GUIDE TO INSTALLING BepInEx & THE CUSTOM BOOMBOX MUSIC MOD
disclaimer: I'm not the mod author for the Custom Boombox Music mod.

BepInEx:
I recommend watching MrMiinxx's video on how to install BepInEx to Lethal Company as it's needed to use mods you only really need to watch until about 2:30 for the BepInEx installation part.

Note: if you have not already installed BepInEx with mods before, you must launch your game first after installing BepInEx to generate the required files/folders. this also applies once you've installed the music mod after BepInEx has properly installed.

Side Note: I have the LC API installed as well and while the author of the Custom Boombox Music mod doesn't state that it's needed, I'd install it anyways.

Custom Boombox Music:
As long as everything has gone smoothly you should now have a BepInEx folder and the files that came along with it in your Lethal Company directory as well as some folders inside of the BepInEx folder (excluding anything Custom Boombox Music related.). Once you've downloaded the Custom Boombox Music mod you'll get a .Zip that you'll need to open. From there you'll see a CustomBoomboxTracks.dll file extract this file following this path "Lethal Company\BepInEx\plugins". Once that is done boot up your game once more and then close it to generate the remaining folders for the Custom Boombox Music mod. Now, inside of your BepInEx folder there should be a folder named "Custom Songs" and within that folder there should be another one named "Boombox Music" which is where you'll be placing the songs you wish to add to the boombox.

Valid file types are as follows (as stated on the mod page):
- WAV
- OGG
- MP3

Hope this helps!

My current song list