I assume you mean "How do I choose which music to play in which level in the levelset I'm creating?"
The answer is musicinfo.conf. Place the music files in the "music" subdirectory of levelset directory and make a musicinfo.conf that contains something like:
Consequently, _all_ artwork should then be configurable using a GUI instead of directly editing the configuration files. Unfortunately, there is no such GUI directly available in R'n'D. Until then, ConfEdit is most likely what you're looking for!
But it wouldn't hurt too much to make simple one-setting per-level music selection GUI? (Or would it?) Think of that and advanced per-element multi-setting graphics/sound config GUI as different things.