Hello there !
I was browsing a little and found a mod that I think would suit Mythic. It's a mod called "Better Records" and it allows users to add their own music and sounds into minecraft. It adds blocks like music etcher, record player, speakers, lasers (Yes some of them aren't needed and can be banned but- not the speakers 😃 The record player really needs them). If it does get into the Mythic modpack somehow, I'd suggest rules for it like "No songs with curses" or so... but that's up to staff- I also sadly didn't find any article about it explaining if it works on multiplayer or not, but it is worth a try.
Anyways, thank you for your time and consideration beforehand 😃 Have a nice day.
IGN : david556
Mod Name : Better Records
Mod version : 1.7.10
Main mod objective : Add custom music