Hi, if you are getting SPDIF over the optical out then the BITS are flowing.
Does any sound at all go to the SPDIF device? If no then you didn't configure output sound to be digital vs. analog. Double click the windows volumn control, click mixer, open the output device list, select digital. Make sure the digital you select is from your SOUND CARD or MB, not from your HDMI video card.
IF you set up digital correctly then your receiver (or whatever is getting the bits) is getting a clean connection, and the problem is it's not 5.1 ? Correct ? IF so Double check your audio sound settings in windows to see the speaker setup. The best way to do this is to run the sound config utility that came with your audio. For Realteck this is Realtek HD audio Manager. For sounder blaster X-fi you use the creative utility. Goto speaker setup. Could be it's forcing the output to stereo. Set to 5+1.
If this is not the solution then please post