i have dvi output on my video card. i use a dvi-to-hdmi cable to connect up to the receiver. i get surround sound on all ss sources.
you will not get ss on mp3 files. it should be stereo (left and right channels).
i'm thinking if you are getting center channel then you have some option tweaked wrong on your receiver or in the audio player. windows sound options should be limited (the receiver should be the default device).
sure you could use optical but you should be able to get it working without. i dont think optical would really fix this problem at this point in time. try everything else first.