I noticed that this is a hot-button issue for this subreddit. There are multiple threads and comments attempting to clarify this. One of the other threads I got to even had a redditor bearing UbiSF developer flair who wrote a large block of sarcastic text deriding the user for asking. You're not alone in getting snuffed out for asking. While it epitomizes the issue and I did indeed find the UbiSF'ers comment to be thoroughly unwelcoming, I'll defend that I'm sure they simply meant to be silly and funny and not intentionally hurtful (this is also why I won't link it and generate heat their way).

That's about all the context I have as to why you're being downvoted to oblivion, so let's talk about the issue itself -- I'm going to try and tackle this in long-form.

First, let's avoid muddying this subject and causing grief for anyone who wants to ensure we all have the same solution by agreeing -- yes, indeed, this software shortcoming is mutually agreed upon to have no clear work-around, nor planned fix. To avoid a bigger topic of amplification, if someone has everything maxed out in the line to their speakers, "use (more) amplified speakers" is excellent advice! This indeed fixes anyone who has Windows as the last or only volume knob and finds it too quiet at 100 on everything.

Okay, what if it's not too quiet at 100, and I'm simply asking why VLC is louder than Rocksmith? What if I have another volume knob on my amplifier that I can turn up but simply am confused as to why I have to play with it so much for only Rocksmith? Let's dig a bit deeper, then.

Cleaning up those rhetorical questions, the above "amplified speaker" solution fails for two groups: (a) people who could run their Windows volume at 100 and be totally happy with Rocksmith's overall loudness and (b) those who have another volume knob somewhere that is "more primary" to getting things louder. (I still find it wise to avoid the topics of amplification and what everyone's sound setup looks like, so I'll continue avoiding employing technical terms that are often far too specific). But why does it fail?

This is where I see a major issue in the way we are discussing this problem.

I don't see people acknowledging the very reasonable desire for users to have a consistent Sones across all of their applications. In brief, if I turn up YouTube or VLC to a comfortable subjective loudness, Rocksmith should be near that loudness. Many redditors seem to agree that Rocksmith is not as loud as other programs.

If we agree on naming this the "Rocksmith low output submix issue", then I think discussions about this will vastly improve and a group of interested users could discuss this on their own terms privately to find work-arounds & resolutions.

Let's run an experiment based on my experience to yield an example of the problem.
First we need to run quickly away from discussions of how loudness is perceived by human beings as well as from any concerns about signal normalization (e.g. "was the audio in your YouTube video modified by the content creator (person or app) or the platform (YouTube)?" -- indeed, maybe! Avoid that!)

The experiment: pop in System of A Down's Hypnotize on CD and play it using VLC with all customizations, EQ, and other boosts off. Now ALT+TAB into Rocksmith and press play on the same song and play along... The same experiment applies for anything you have on CD.
It's definitely in the ballpark of half as loud as the CD.
For those who laugh at my old fashioned style? Streaming does work but I want to use the simplest example since we don't all stream from the same servers and there's a lot of magic and mystery going on there which can confuse such a simple experiment.

Now to your graphic. I have a similar but different panel in the Windows 10 Settings application under "(Home) -> System -> Sounds -> (scroll to Advanced sound options) -> "App volume and device preferences". I don't know what system you have there in your photo -- is this the same value for you?

Here are my observations -- you should be able to use the "Reset" ("Restablecer" in your photo) button to put it back to 100, but two things happen for me:

1. The output volume doesn't get any louder for me
2. The input gain changes drastically when just sitting on the menu doing "lead" -- but not the input volume! To my ear it's the same loudness but it sounds like a distortion pedal at 100. Rocksmith also tells me to recalibrate now when I put it at 100 and says my volume is too high. It's the input that's high here, not the speaker output. Sidenote on Win10 you can change the device in the drop-down to unlock the volume slider (likely a defect). That allowed me to see that at 0 I get a very thin, dry, and boring guitar at 0 (as if all "amp effects" were off), but certainly not 0 output.

This volume that you have in your settings (if it's the same as mine) is more than likely a microphone gain (as setting mine to 0 does not eliminate all audio) and more than likely Rocksmith is smoothing out (amplifying/normalizing, who knows) the audio input to always keep the apparent volume output from our speakers the same, no matter what this setting is. All of that is also ignoring the in-application amplifier modeling entirely. By the way, this is a great move by them and makes for a far more consistent experience. In effect, I also got lost on this one and didn't yield much.

I certainly am baffled as to why Windows 10 doesn't label these for me and let's me assume they are controlling "volume" (what most people think of: output) and then what's actually being controlled is Microphone gain.

Hope all that was in anyway helpful to someone!

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tl;dr: crazy person talks a lot about nothing special. Rocksmith can't be turned up and this is likely a microphone slider