Possibly related, but in Windows for me - for years now - the Sound control panel and nVidia sound driver, always on reboot of the PC, reset my channels to 2 channel stereo. I have to go back in after a reboot and reconfigure for a 7.1 setup.
Don't know why this happens, but it is sure annoying and has nothing to do with Media Center or any other app. Just simple reboot and it happens. This is accessed from:
Sound (Control Panel) -> Marantz AVR (nVidia High Definition Sound) -> Configure, where it will show "Stereo" and I think need to re-select "7.1...."
Now Media Center never changes my setting in DSP studio - it's always "Source Number of Channels" for my Marantz Zone, regardless of that Sound control panel setting. Games on the other hand are always affected. Note to the Marantz I always bitstream audio so perhaps there's a difference there as well. For my USB DAC and associated zones, none of this matters. Since it bypasses the nVidia driver, the control panel app, and the Marantz and uses the ASIO provided driver from exaSound.