Don't know what it could be. I googled around and it was to no success except i think a lot of other people are getting this problem. I'm running Windows 7 64bit Ultimate. Newest drivers etc as far as i can tell.
I installed the OS yesterday & when i have to come to play music it was playing in stereo. So i changed the options on Realtek HD Audio manager to 5.1 and same with control panel, this has seemingly done nothing. The part puzzling me is that when i click 'Test' it plays a sound through all the speakers, so the speakers are connected right & it should be setup right but it's not working :s
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Have you tried changing the default device by right clicking the Realtek icon and choosing 'Default device' and changing it from analogue to digital?
If the (Music/game) was recorded in stereo, then you will hear stereo. Have you tried with 5.1 encoded source material?