I wouldn’t say that this one is crazy, but it has been inspiring while playing trem at times:
I don’t play music most of the time while playing trem, as I usually focus on the sounds of the game while I play for more information on what is going on around me. I do play Xonotic on occasion, and most servers have music playing in-game (with the option to disable using the radio commans). There are default tacks (which I believe are licensed gpl, although gpl is not the best free as in freedom license for assets, Creative Commons is an example of a better set of free as in freedom licenses for assets),and various modded servers provide many additional tracks (although idk about the licensing of all of those tracks, nor if they all could be freely distributed legally).
Tremulous’ engine does support playing in-game music (at least it looks like the basics are there from the ioquake3 engine), and if a/some musicians were to create awesome musical tracks that fit in with Tremulous and were licensed to be freely hosted, distributed, and played by anyone, such tracks could be provided as an option in Tremulous.
Another possibility to consider is a concept that @romdos mentoined was that in another game (I don’t remember which he mentioned) the sounds of various game items/entities were like musical components where together they would create an atmosphere that was musical while still communicating useful information about the game environment.