youtube/pandora/windows are stereo sound only so thats normal.
in order to allow sound to come out the back speakers there is typically a setting in your soundcard/onboard sound driver software to allow for this. this may vary by what brand you have.
keep in mind you will need to enable/disable this every time you want to switch back and forth between true surround sound 5.1 and just having the rear channels copy the front channels.