Hm, can actually setup a steam discussion for figuring this out - this shall suffice as a basic tutorial.

Yeah, that's quite an adventure at the moment to add anything custom if you are not already intimately familiar with FMOD.
This mod is a part of something much bigger and that something will come with a detailed documentation that will include several guides - custom music for Marshall tutorial is considered too. Right now, however, the best I can do is direct you in a personal manner on the ways to get it done (it's some hustle for the initial setup but once completed, adding more is a breeze) in Discord or wherever.

and how would you do that aswell, i have no prior experience with something as a BANK file type

so you can modify this pack to add your own music?