Have you unplugged the TV and the Beam from the wall for 5 minutes, then plugged them back in? It’s unfortunately a different result than the simple powering off procedure. 

Yes, tried unplugging several times. Left both unplugged overnight, just to be sure… no change.

Hmmm. That’s definitely odd. My guess is an issue with CEC not making the appropriate connection, which fits the facts as you’ve stated them. Can you stream music to the Beam?

I would really be tempted to contact Samsung’s customer service first, and then Sonos’, although Sonos, being a trailing and relatively simple device, seems to be an unlikely suspect. My money would be on the circuitry in the TV set. 

In fact, I would be tempted to double check the settings in the TV, both to ensure it has the latest OS, and that for some odd reason, CEC hadn’t been turned off. 

Yes, I can stream music to the Beam (which is Airplay 2 compliant) with no problem.

I contacted Sonos customer service, and they helped me bypass the CEC system and use the optical connection instead. So at least I now have sound -- although it’s nothing like the Dolby 5.1 I was getting before. 

The Samsung TV is almost 10 years old and it seems they don’t support it any more. Sigh. It has the latest OS, and HDMI-CEC (which they brand as Anynet+) is turned on; everything is set exactly as it was before it stopped working. 

I have a strong hunch that the issue is actually the Apple TV box. I’ve seen various posts online that the 15.6 update and 16 beta break HDMI-CEC for some people. No words from Apple, but they generally don’t tend to be upfront about that kind of thing. Given that everything was working fine, and suddenly stopped, without any changes of settings, I think this may be the culprit. 

At least I have a workaround for now. Thanks for your thoughts!

Huh. I’ve not seen that issue with either Apple TV I have. But it’s hard to argue. Good luck!