Optical tp pc problem

I recently connected my Sony 5.1 surround sound receiver to my new pc via the optical cable. However the problem is that with most things I play (mp3's, mpegs, etc) it makes the receiver only play in stereo. That's what it shows on the receiver screen, and sound only comes out of the left and right front speakers. When I play a DVD or Blu ray then it switches over to 5.1

How do I fix this? I want to be able to play music through all speakers in the room, not just two.

Any advice would be greatly appreciated

 

It just seems to auto decide what to do. Been messing with it all morning with no solution

 

A days gone by and no advice. Can anyone steer me in the right direction??

 

I can. You will have to select a sound-field that expands the sound to other speakers on the receiver itself. Also, Foobar2000 music player has a plugin to auto-expand stereo sources to quad. Hope it helps.