exactly
Humans = Metal (Finnish Death Metal, ftw! Swedish Sleaze!!!111!!!)
Aliens = industrial electronica goth glam punk garage dark and screeching or instrumental horror flick music (synthesizers thru marshall amps)
Music should emanate from main structure (reactor/OM) when it is at 100% health
Volume should decrease with structure's health and distance from it
Then the enemy could never hide their base

And campers would never move ("They're playing Tool/Korn/Pantera, man. We can't move, now.") :roll:
But seriously, which team you choose to be on could influence which music you are listening to. Humans could listen to Metal. Aliens could listen to Punk/Electronica/Industrial/Goth mix. Players listen to music, and game sounds, in clientside interface with game volume and music volume selectable at options menu. Humans get one mix; Aliens a different mix. Rockers v. Mods, anyone?