All of the in-game music is on the Resources tab and yes, they have to be named music_filename. That's where and how I set my music files up for my mods.
I didn't use the Sounds tab at all when setting up music files. I did use it for a map once, by making a music file a sound file and having it be a global sound on the map (but that was before I started the mods, a few years ago).
Dana