Have you tried turning off HDMI-CEC?
Thanks for the suggestion, but the audio issue apparently was just the symptom of our receiver kicking the bucket. The receiver developed other issues in the next two weeks, including screen freezing, a zombie-like state where it would respond to nothing but unplugging, a 981 error message boot recovery, and continued random freezes and reboots.
Dish sent us another 722K which has worked fine so far.
One thing I learned is that it is best to call tech support close to 8:00 AM Eastern. I actually talked to someone in the US who could actually help. My worst experience was a 9:30 PM call where I was on hold for 30 minutes, and then had to talk to someone with a heavy accent in a foreign call center. My inability to understand her made the call less than fruitful.