[Warner306]

So, brazen I think I had a bug similar to what you were describing with new Nvidia drivers. After a power outage, my AVR would stick in multichannel after playing a multichannel source. It would also output all stereo sources in multichannel. Other codecs would switch the AVR into the correct mode, but it couldn't handle multichannel sources.

This is the single most irritating I have had happen to my hardware, and it's even more annoying when it's caused by a power outage. 

Here is what I tried:

• Cleaning and reinstalling the same driver as well as a much older driver;
  
• Switching from WASAPI Exclusive to WASAPI Shared, which changed the behavior but didn't fix it;
  
• Resetting the AVR to its factory defaults;
  
• Running the Windows audio troubleshooter.
  

Before attempting a system restore, I selected Nvidia (High Definition Audio) as the audio source and played some content including a video with a multichannel AAC audio track. I didn't get any sound with the AAC source. I switched back to the AVR as the audio device and the problem seem to disappear.

I still don't know if this fixed it, but the problem is likely resolved. It was like something was saved in the memory of the AVR or Windows and this was somehow reset when I selected another audio device on the same HDMI connection. It couldn't be the EDID, as I'm pretty sure it is erased when the driver is uninstalled.

I have doubts this will fix your problem if you encounter it again, but thought I'd pass it along anyways...