I had issues with my Windows XP system from time to time resetting/reverting the speaker settings back from surround (4.1) to desktop speakers.
I finally found a newsgroup posting that put it in connection with my Creative Audigy 2:
Some games and other programs can change the Windows speaker-settings though, so you might want to try the second solution: stop the Audigy from using the Windows settings. Make sure you have the newest drivers installed, go to the Audigy 2 speaker settings program and uncheck ‘Synchronize with Control Panel’. That’s all – the Audigy 2 should now remember it’s own speaker settings, insteaf of resetting them to those of Windows.
Downloading the latest drivers from the soundblaster.com site also included the Creative Audio Console, which can do what the posting above points out. And sure enough, I suddenly have my surround sound in World of Warcraft again.