Give me a great selection of classic music split into "event directories" to follow previous theme, then a programmable system for users to enhance the player itself. Like:
1) Continue previous track when current event has ended, rather than restart or start another one.
2) Being able to detect "visual mode"; if I want to have "cockpit speakers" the two channels should be "localized in 3D space" and react to head movement. If I want "cockpit headset" the sound follows my rotation (no change in sound). If I enter a space suit, maybe I want the sound with me? Maybe I want the possibility to automute any music playing while in 3rd person to enhance my realism feel? In general, highly programmable through a scripting language.
3) Crossfade speed between tracks.
4) A good selection of volume (and stereo balance) controls. Some games only have music on/off - completely unacceptable!!!!!!!! That forces me to play without music when I know they have good music but too loud for my liking.
5) Customizable sound mixer (if at all possible, dunno): Music out this port, voice out that port, and sound effects a third port. I.e. cockpit speaker music to my front speakers, voice to my headset, and sound effects through the surround system.
Guess I'm more interested in the "sound technology" used in the game than in the chosen tracks. As for track selection alone; as long as I'm able to put in whatever I feel like easily from my own collection, I'll be happy.