Playing stereo mp3s through 5.1 channel surround

[NoSmokingBandit]

I just got a 5.1 channel system hooked up but i cant seem to get my stereo mp3s to play through all the speakers at once. In Audio Midi Setup i can choose either the rear 2, front 2, or center/sub, but never all at the same time. This was an easy thing to set up in windows, so i must be missing something obvious here...

I would be easy if Audio Midi Setup would let me put channel 1 and 2 on more than one speaker, but it doesnt...

 

[sammich]

I'm assuming you've got that system hooked up via optical. Does the 5.1 have any setting to play on all channels?