most music is recorded in stereo (left and right channels).
the front 3.5mm port sends a signal to the left and right speakers (channels).
quite a coincidence!
unless you are playing surround sound songs or surround sound game/video audio i dont think there is anything wrong.
it is perfectly normal to hear your music on a surround system through just your sub and front speakers. now, you can change that in your audio properties to a different mode which causes the rear speakers to repeat the same audio as the front speakers but you aren't really "hearing more" just pushing it through more speakers.