This is what I've run into as well setting up a new HTPC.

Win 7 x64 home premium RTM.

Using the HDMI out of the video card to a 7.1 reciever, only using 5.1.

Configured Windows sound to use the HDMI out on the video card, configured speakers in the control panel sounds for 5.1

The test for the speakers works correctly for all speakers and sub.

Both Windows Media Player 32 and 64 bit versions play DVD's correctly with 5.1, receiver shows Dolby D.

Windows Media Center is where the problem comes in.

If I play a DVD in WMC the video gets choppy and crashes with an error of missing / problem video drivers or codecs. Should not be a problem because both WMP versions work correctly.

WMC is defaulted to 2 channel speakers, and everything out of it is in all channels stereo.

If I change the speakers settings to 5.1 and test, it only plays sound out of the left front speaker. At this point it does not matter if I select pass or fail to the test, WMC no longer plays sounds until I close and reopen. Speakers reset to 2 channel.

If I change speakers to 7.1, there is no test option and there is no longer any sound unless I restart WMC. Speakers reset to 2 channel.

To get things working I had to set the speakers to 7.1 in the control panel. I disabled the missing L/R side speakers.

Now I can set WMC to 5.1 or 7.1 and sound does work. Receiver shows Dolby digital instead of stereo.

DVD's now also play instead of crashing in Media Center.

Beats me what is going on here, but this seems to have solved it for now.

Now if I can only get Media Player to stop randomly loosing album art, another seemingly common complaint...