Short description: I made new music for the game because I found original music too loud when I listened to something else while playing. ..and wanted to have some music in case I don't. Music is properly mixed and mastered, and nicely packed in to a .brz-file. Although the file is about 30 megs, I think it might be worth a try.
You can get it from CMMODS, just search for Kerko's Alternative Music.
Long description copy & pasted from the readme:
Replaces all three music files played in menu screen, while loading mission, and while viewing end of battle screen. Pieces loop seamlessly except 'music end of battle.wav' which is only played once.
Music is dark, atmospheric and mixed to be soft enough to not to annoy you even if you like to listen to your own music while playing. They are all original recordings especially made for this game. Do what ever you want with them.