Shoutbox

I think that's because most of the things you play like mp3 are stereo and you have to tell your surround software to mix stereo up to 5.1 else no sounds comes from the back.

I had the same thing with my 6.1 Creative Audigy set, but that has a feature called CMSS 3D and when I enabled that the sounds get mixed up right.

Maybe your surround software has the same feature or something like that.