I noticed it last week but only tonight have I tried to sort it out. I have SB Audigy(standard)and normally my 5 in 1 spkrs have worked in music and games now for some reason they only work in games and the centre spkr doesn't work when I play music cd. Test speakers they all work great. OS Win XP.
If you have your sound card set up correctly then you should hear music from the left and right speakers only. [Stereo] + the base speaker even if you are using 5.1 - 6.1 - 7.1 speaker systems.
You can play about with the settings to make the Sound of Music [Isn't that a movie?] come from all the speakers but it will still be stereo. Some music tracks are now available on multi channel tracks but its mainly games and DVD movies' that use more than two tracks.