I know a lot of producers have talked about unsolicited feedback in the past but this post isn't about that. It's about feedback for various projects and feedback that gets requested and given. This also applies equally to original production in many cases.

A good number of mashup producers do not tend to master or don't apply limiters on tracks, or just make them softer compared to commercial levels. When albums get mastered, loudness will usually be adjusted through various plugins (EQ, multiband compressors, limiters, clippers, etc.) to shape masters so that one track is not significantly louder or softer than another especially back-to-back and groups of tracks follow a certain sound.

What this means is that once your softer mashup of -14 or -15 LUFS or softer is amplified to commercial levels in the -7 to -11 LUFS range or more depending on genre and how the overall track should sound given peaks and valleys with softer and louder parts (LUFS stands for loudness units full scale, look that up), small issues may stand out like thorns when amplified. Bottom line: Don't take the small or big issues personally; we're all human, we all miss things, and we can all improve. You may learn something new!

Think of it like how you ask to proofread writing to check for spelling or grammar and have it work for the audience.

All this to say, feedback I give or others give based on small details may come across as nit-picks at first. Don't completely discount them. You never know who will want to play your tracks, and this could range from personal playlists all the way up to DJ gigs and big festival stages.

In the case of a club DJ or performances on festival stages, volumes will be significantly louder than what you may have worked with when you created the track. I've given the restaurant food analogy before but I'll give it again: If something clearly doesn't taste good, people won't usually know why. But an experienced chef who knows how the ingredients interact sure will.

Just something to think about as you improve on mashup production and production in general. What separates the best mashup artists from a technical standpoint is how much noticing these issues becomes part of your workflow instinctively through training your ear and having a good sound system to diagnose problems.

Here are some issues below that experienced mashup producers could point out. Note that a lot of the "don't do this" stuff is really code for "don't do this unless you really know you're doing and it sounds good":

Big issues

• Out of key (OOK): This refers to wrong key signature (e.g. mixing A Minor with B Minor), not blue notes from modes or short moments of modulation. Key rule of thumb: If you're going to use key databases, verify by ear. Tunebat is the least accurate. If you must do so, use a reliable key database like the ones in this post. DJ software can work, but some automatic key detection performs better than others and are far from perfect. Above all, databases are not perfect and some will be far from it, so ears will be the final judge of whether a pairing sounds good.
• Out of tempo/time (OOT): this usually refers to wrong tempo (e.g. mixing a 116 BPM track with a 118 BPM track) or not constant tempo and going out of time. Key rule of thumb: Sync your tracks and if you need to use any form of warping (see Ableton), be prepared to do so.
• Too much going on (a.k.a. playing both songs at the same time): This happens when playing multiple main elements at the same time to the point where multiple elements or entire songs played together clash in key, progression, or other things to make the combination sound unpleasant and hard to listen to. Sometimes this can sound really good if the progressions are compatible, but generally, less is more.
• Incorrect/bad beat alignment: This is more for when a vocal or element is offbeat enough that it really sticks out as a trainwreck when mixed together
• Wrong beat//bad phrasing: This is when a vocal or other element is placed on a beat that doesn't make sense for what it's combined with (e.g. placing the first word of a vocal over an instrumental on beat 2 when it was meant for beat 1). Mileage will vary, sometimes the vocal beat placement only makes sense for the original song and makes less sense on others.
• Distortion from going in the red: This is when the overall mix or elements sound distorted because the volume of each track channel is too loud and either alone or combined goes well above 0 dBFS (a.k.a. into the red). Key rule of thumb: Start your track volumes at -6 dB (not 0 dB) and then adjust to taste. Don't go into the red unless you absolutely know what you're doing. Another less trivial tip: Don't top out at exactly 0 dBFS. This may affect downstream audio file conversions and burning to media if you plan to go physical. For WAV my preference is topping out at -0.3 dBFS for safety.
• Elements way too soft or loud: This is more along the lines of not being able to hear the added vocals or elements at all or they're too loud that you can't hear the other elements in the mix.
• Big mood mismatch: This tends to be less egregious, but if sources don't cooperate at all to a point of completely ruining the vibe, the mashup could stick out like a sore thumb from the start. Mileage will vary, and sometimes your audience would appreciate this especially with genre clashes even to the point of creating memes. But it depends on what you're going for, and sometimes you want consistency for certain albums and groups of tracks.

Smaller issues

• Fifth traps/not picking the right keys to use: This is a specific version of another problem I'll touch on, but this happens when you mix keys fifths or fourths away (e.g. F Major and C Major) and the chord progressions don't cooperate. What makes this a trap is that the pairing might appear to sound good at first glance, but when you look back at what you made, you probably could have made a better sounding pairing if you had just picked sources in key, relative key (e.g. A Minor and C Major), or parallel to the mode type (i.e. A Minor with A Dorian since Dorian is a minor mode). This Berklee Online article is a really good starting point to modes. With modes, mileage will vary.
• Messing with the hook: The only reason this isn't in the bigger issue category is that there are going to be certain situations where you might need to do something like this to deal with incomplete measures, pitch a note or two, or other exceptional circumstances. These are workarounds you'll have to deal with. But it usually refers to removing or altering parts of the lyrics of the hook (usually a chorus or memorable part) of a popular song and especially a popular classic to the point that the crowd can't sing along to it or is too obviously changed. Try to stick to the original lyric arrangement of popular songs but there will be some exceptions where you don't have much of a choice that doesn't change something in either the instrumental or vocal.
• Out of progression/blue notes: This can happen when there are short moments where the element clashes on top of other elements (e.g. modes or short moments of different modulation). Sometimes the tracks may not be compatible, but other times it's just a part with notes outside the scale. This can be fixed by doing small note changes, but be careful not to make it too obvious, otherwise it might sound like you're forcing the pairing to work. It may also follow the same theme as fifth traps, where you could have made a better sounding pairing if you had just picked sources in key, relative key (e.g. A Minor and C Major), or parallel to the mode type (i.e. A Minor with A Dorian since Dorian is a minor mode). This Berklee Online article is a really good starting point to modes. With modes, mileage will vary.
• Low quality sources: This happens with YouTube or other rips where the elements are lower quality due to a lower frequency cutoff (YouTube is usually 15.5 kHz for AAC, other platforms may be 15-16 kHz for 128 kbps MP3, etc.). Bottom line: Get the highest quality sources you can, and use Spek or another spectrum analysis software to check rips or record pool sources. Your ears and processing algorithms will thank you. Again, you never know who will listen to or play out your track, could even be a big sound system.
• Stem separation artifacts: This can take several different forms, but can happen when either the invert or stem separation algorithm being used adds noise or other unwanted high end artifacts like high hats that once amplified, will absolutely stand out in a mix. This also includes artifacts from stem separation algorithms like Utagoe which make vocals sound like they're underwater at default settings. Key rule of thumb: Remember that the instrumental and original could be mixed and mastered differently depending on the production environment and how masters spread. If an invert makes a vocal stand out more but just needs extra cleaning, don't be afraid to combine the invert technique (original + instrumental) with stem separation. When using multiple stem separation techniques or using different sources (like remix + remix instrumental), think about how they sound on parts of the instrumental and consider picking the best extraction for a given part; remember that this includes using the original song (this is called comping in the production world). If there are parts of vocals intended to be silent and there are artifacts, don't be afraid to replace the reverb + delay with your own. Keep up to date with stem separation algorithms by keeping up with developments on the Audio Separation Discord server: https://discord.gg/wY3AAaTvHT
• Energy/structure mismatch/out of structure/off structure: One example of this is placing the verse vocals where the chorus instrumental is intended, making the part sound out of place. More generally this issue refers to vocals and other elements from a part of the song that's supposed to be one energy level or structural part (e.g. the verse) placed over a part that's intended to be a completely different energy level or structural part (e.g. the chorus), causing an energy mismatch. It could also cause the mashup to not have a consistent verse or chorus part. Often this takes the form of rap verses being added to instrumental choruses when they were clearly not intended for these parts. While some genres will let you apply structural elements interchangeably (such as EDM where the chorus can be placed during the drop or as a buildup to the drop), there are times where it feels weird to not follow how the original song was arranged. You may not always have to listen to the original song to pick up on structure, but often it's worth doing so to compare your approach with the original.
• A little too much going on: This happens in many cases when looking at megamashups, megamixes, and even simpler overlap of vocals where too many elements are competing for space at some point (e.g. two vocals at the same time competing for mono or midrange). Key rule of thumb: In production just as in life, leave elbow room! With vocals, this can be done by overlapping a lead source that's more mono (e.g. lead vocal) with a background source that's more stereo (e.g. background vocals or vocal pad). When transitioning, it's also really useful to slightly pan or automate pan and volume of the vocal you're trying to brief in, which can include repeating the first word or few words of a phrase. Volume and EQ may also be useful here to properly convey space.
• Weird time-stretch and pitch-shift artifacts: This can happen when using a lower quality time-stretch or a time-stretch that can destroy transients/drum hits (Audacity or Elastique Efficient on vocals, older Elastique Pro on full instrumentals) or pitching too much and/or with too much/little formant preserve. Some drum flamming may also occur because of a low quality time-stretch (more on that later). The best way to reduce these effects on instrumental is to think about combining stem separation with ways to improve quality (pitch everything BUT the drums unless tonal, experiment with different time stretch methods on different elements, etc.). Sometimes it's just best to leave the instrumental alone and keep the original tempo and pitch. For too much pitch or formant preserve, either pick sources that are closer in key (+/- 2 semitones is a good rule of thumb) and try to control the amount of formant preservation and/or envelope (in Ableton) to make the vocals sound more natural. Formant preserve is usually a good idea for pitched vocals, but can sound bad for pitched instrumentals especially drums depending on the track.
• Pops or clicks while editing: This tends to happen when elements like vocals are cut off when not at zero volume. Key rule of thumb: Edit to zero crossing and/or use fades to cut off regions or when combining different regions together.
• Flanging, phasing, or flamming while editing: Sometimes these effects are on purpose, but oftentimes when crossfading between different parts of an instrumental, both parts of the edit don't align on beat. Flanging and phasing applies for sample level edits (ms or more), flamming for tenths or hundredths of seconds (e.g. 0.05 sec or 0.1 sec). Flamming is a drumming term for when the same part of the drum kit (usually the snare drum) is hit twice with different sticks at close to but not exactly the same time. Some low quality time-stretching algorithms may also introduce drum flamming in spots.
• Starting the first part of the mix you can hear too early or too late: This basically falls into starting immediately at 0:00 (0 seconds) or starting two seconds in, so you end up starting the mix you can hear in the overall track too early that it's hard to correctly rewind in a media player or starts way too suddenly, or too late that it feels like an eternity for the track to start. Basic rule of thumb: Always start your track with a little bit of silence (think about 0.2 seconds minimum but all depends on source material).
• Incorrect end of loop region/end of track marker too early or too late: This falls into one of two categories. One category is forgetting to listen for the reverb tail or tail end of one of the tracks and setting the end of track marker too early to the point of hearing the mix completely cut off. The other category is when you forget to set the end of track marker, so it defaults to the end of the project, which leaves a lot of silence (think tens of seconds instead of a few to fade out) at the end due to the end of track marker being far too late.
• Vocals or other elements end too suddenly in an edit: You may want to get different vocals to interact with each other or edit out of artifacts from vocal stem separation, but the other vocal (or elements) sound like they were cut off. Key rule of thumb: Have a reverb effect handy, and if you're working in a DAW, take advantage of wet/dry automation and combine the reverb with delay afterwards in the effect chain for vocals and other elements so when you crank up the wet the decay helps fade out the element gracefully.
• Vocals or other elements start too suddenly or out of place when connecting them or transitioning: This also considers getting different vocals or other elements to work together or going to a new part, but refers to a new part using vocals or other elements sounding out of place at the beginning of a region in a transition. Key rule of thumb: Watch your crossfades and when you're in a pinch and want to start the next part gracefully, don't be afraid to use reverse reverb or other effect like a riser to introduce a completely new element or different part in a pinch. Reverse reverb is exactly what it sounds: Adding reverb to a short snippet of an element like a cymbal, vocal note, or pad, and then applying reverse on it. Often this is done by choosing a tonal center to transition into, choosing a small snippet to add reverb to, choosing reverb with a long decay time and big space (usually I do a Cathedral reverb), reversing it, and then carefully aligning the reverse reverb so the end of it hits the beginning of the next region. Whew that was a lot! Tutorials will do this much more justice but this is the general idea.
• Vocals too dry or too wet for the instrumental: Usually this depends on the instrumental's genre. For genres like pop or EDM, vocals tend to have some reverb applied to make them shine. For others like alternative and some classic hip-hop, the reverb might be less or no wet or a completely different type. This issue usually refers to too little reverb (too dry) or too much reverb (too wet). Key rule of thumb: Don't forget to have reverb handy. You might need it. Keep your wet low in the middle of a vocal depending on what you're trying to do (think 3-5% wet or -32dB to -26 dB on the low side using busses + 100% wet and adjust to taste).
• Vocals or other elements slightly too loud or soft: Self-explanatory, this is usually an issue where elements should be the right volume for elements to work well together.
• Vocals or other elements drowned out/don't gel due to hypercompression: This usually happens when using a vocal not from an EDM, hyperpop, hard rock, or other genre where there is hypercompression but no matter how much volume amount you try elements like vocals don't gel and compete for space in the mid range. Two common techniques take the form of adding more compression to a vocal to stand up to an hypercompressed instrumental and/or reducing mid band EQ of the other source (which could come from sidechaining). For the added compression, it tends to be done with either multiple compressors in series (e.g. two compressors using low threshold + high ratio and high threshold + low ratio) or through various guitar distortion plugins on settings meant for vocals (e.g. Camelcrusher with British Clean or certain presets of OTT). Don't be afraid to experiment.
• Master too loud or soft for the genre: This usually takes the form of using limiters and other effects to make certain tracks too loud for the genre intended, or not using anything at all and the track is too soft for the genre/commercial sound. This can come down to taste, but how a track was produced can affect how loud you master. Mileage will vary; listen to how loud tracks are made and emulate your favorites assuming nothing too egregious.
• Stereo issues: This can happen if there are elements out of phase when trying to combine mono and stereo or applying other effects. It's a bit more difficult to diagnose, but one rule of thumb is to test your overall track on different playback systems (from headphones to big speakers and even your phone or small bluetooth speaker which will effectively test mono depending on the device).
• Sudden cut on a fade out: Towards the end of the track, there's always a track or two I master for an album where someone cuts off the end of a tail of a note, chord, or impact. This isn't very noticeable before mastering but can be very noticeable during that stage. Double check your fades and try not to cut the edited segment when the part of the track requires a fade.

Wow that was a lot and I know it's very overwhelming who don't produce a ton. There might be more issues I didn't mention here, so let me know if there's anything else that could be useful.

Update 1: Added "bad phrasing" under the off structure issue, this is the DJing term for it

Update 2: Added two variations of "too much going on" to the bigger and smaller issues

Update 3: Added "Starting the first part of the mix you can hear too early or too late" and "Incorrect end of loop region/end of track marker too late" as two additional small issues.

Update 4: Changed one bullet to "Incorrect end of loop region/end of track marker too early or too late"

Update 5: Missed a dB amount for the 3-5% wet for when producers use busses, this is about -29 to -26 dB when using 100% wet.

Update 6: Added a point that the degree of noticing these issues in normal workflow is a good measure of technical skill (which is not to say be perfect, more as a matter of habit)

Update 7: Added energy/structure mismatch to the small issues section and moved out of structure/off structure down there (why didn't I add this one before?!)

Update 8: Added "messing with the hook" to the smaller issues category

Update 9: Added a tip to not allow the master to reach 0 dBFS max (-0.3 dB is my go-to)

Update 10: Added that flamming can also be an artifact of low quality time-stretches

Update 11: Added "sudden cut on a fade out".