Is it possible to add custom music to the game?

1. Yes, approximately like that. But if you delete the paths in the files (Music), the music should stop playing!
2. Converting one file to wav won't take up much space, and neither will 10 (relatively). I don't know who told you that the map won't launch if the size is too large... In my map that I'm preparing, I exported about ten models and a lot of textures, and nothing, the map launches without bugs.