I had a run into the Sauerbraten Game Engine, a spinoff of the Cube Game Engine, made for simple MP Deathmatch fun.
There are loads of costum made music in there.
http://fanaticalproductions.net/ offers some of the nice stuff!
To sum it up with my uneducated words:
"Duh, sounds a lot like a Quake 2 soundtrack but not that bad either!"
