I've spent half a day trying to resolve this, and haven't had any luck. Here is what I'm trying to do:
Hook my PC to my 5.1 AV Receiver and be able to watch MOVIES, DVD's, and play video games in 5.1 SURROUND sound. In order to do this, I had to connect the S/PDIF_IO on my motherboard to my GPU, then plug in a DVI to HDMI adapter to run the signal through HDMI to the Receiver. My PC can detect the receiver and I have it set to play audio through it as the default(it shows up as High Definition Audio Device). The only problem is that it only plays audio through the left and right speakers(it also utilizes the subwoofer). It won't let me click on "Configure" in the Sound category in Control Panel, so I can only go to Properties. From there I can run the audio test for DOLBY Digital, and all of the speakers will work properly. I can't understand why the 5.1 will work when I run it through the test, but nothing else seems to utilize anything more than the left/right speakers.

My current setup:
Windows 7 64 bit
GeForce GTX 260(which I believe is capable of OUTPUTTING up to 8 channels)
AMD Phenom xII 1100T Six Core ~3.3Ghz
MoBo: Gigabyte GA-MA78LMT-S2
4 Gigs of DDR3 RAM

This is hooked up to a Yamaha RX-V371 AV Receiver via an HDMI cord to my GPU. I don't have a second HDMI cord at the moment, so I have a second cord going from my second DVI slot directly to my TV via an HDMI adaptor. Are these by any chance just MP3's you are playing? MP3's are 2.0 channel only. Some sound cards/outputs allow the sound card to upmix to 5.1. My card does, but the sound card cost $180 NZD it also has optical out on the card itself. Have you tried an actial 5.1 channel DVD with something other than Windows Media Player? VLC, Power DVD ect. Note you may have to set programs like powerDVD to 5.1 mode.