When I was using analog 5.1, I couldn't get WMP to output to the rear speaker's either.
When I watch a 5.1 movie I get all 6 channels from WMP. When I play the .ac3 sound file for the movie it ignores all settings and downmixes to 2.0.
This actually drove me nuts for two weeks. I way trying to test my SPDIF passthrough using a 5.1 .ac3 test file & WMP.
The ac3 file was ripped from a THX speaker test so I assumed that no 5.1 from WMP meant my system wasn't configured properly.
I gave up and watched a movie and imediately noticed I was getting real 5.1 sound!
Its the same file, but it seems WMP is hard coded to ignore all system settings and think music = 2.0.
If the original is only in stereo and you use a Home Theater Reciever its better just to have the reciever turn it into 5.1. Which is how I solved the problem.
Otherwise would I would abandon WMP and maybe give Winamp a try.