Soundcard settings

I’m running an MS-6163 Pro 600 mHz PIII with 128MB RAM, Riva TNT2 and Vibra 128 sound card with AON2000 20W subwoofer speakers. In the multimedia settings in Control Panel, I set it to Surroundsound Speakers and click Apply, but as soon as I start playing any music, it reverts to Desktop Stereo Speakers. Any ideas why this is happening?