(most) music is stereo sound only and not surround sound. this means that it will only play out of your front speakers.
the only way to get it to play out your rear speakers is to use your speakers in external stereo mode. i'm not sure what the option is called in your soundcard drivers but it basically duplicates the front audio to the back. left front audio is played on LF+LR and likewise for the right. its not going to be true surround sound at all but it does make sound come out of all your speakers.
you will of course need to disable this effect if you want to play a game or take advantage of surround sound again for programs which output it.