It used to be possible in older versions of Xonotic and before it Nexuiz, but for whatever reason I cannot get anything other than stereo sound to work. 5.1 and 7.1 is an absolute no go. Interestingly this was tried with two different soundcards (Asus Xonar DS and Auzentech X Meridian 7.1) under Windows 8, Windows 10, and several flavors of Linux. Even more interestingly it also refused to work correctly under Rexuiz, leading me to believe that this is a DarkPlaces issue.
ECKZBAWKZ HUGE LIST OF ACHIEVEMENTS GOES HERE....
Oh wait.
Oh wait.