I did some digging and found this in the manual for a Philips PLF series manual:
"Due to legal requirements, it is possible that with certain copy
protected PCM audio qualities from a accessory device connected to
the HDMI input, the digital audio signal is muted."
It's possible the 622 is flagging the PCM audio in the HDMI stream as copy protected. Or it's possible the TV assumes that
all PCM audio is copy protected.
It would be interesting to try another HDMI source if you have one (like an upscaling DVD player or Blu-Ray player) that you can use to generate a PCM audio stream over HDMI.
I can't believe
all SD channels would copy protect their audio so maybe it's how Dish Network is originating the program. Or it could be a bug in the 622.