The music is only encoded for simple stereo, not 5.1 surround like a movie.
Because of this, you're probably only getting music through the primary stereo channels (front L/R).
Two solutions, and only one is guaranteed to work:
1. Check the settings of your music player. There may be a setting to upconvert 2-channel audio to 5.1 audio signal. Not all music players have this capability. Sometimes it's something buried in the driver settings. It may not be possible at all with your setup.
2. Run simple stereo to a piece of equipment that will separate channels to allow you 5.1 for any audio source. This is the guaranteed fix, and what I run in my truck.
I've got simple stereo from the PC to an AudioControl Three.1 equalizer and it outputs 4+1. There are others that will output 5.1, which is you want.
Sorry to say this, but the guaranteed fix is also the most expensive and complex, as it adds more wiring and another piece of hardware to babysit.l
Good luck, either way.