Maybe a TV firmware update?

Try disconnecting the Arc's HDMI, thenunplug the Arc and TV for a couple minutes.

Power up the TV and when fully awake reconnect and then power up the Arc. (Hope I got that in the right order)

I gave that a shot, but no luck. Very strange. Only happens sometimes.

Next time it happens, submit a system diagnostic within 10 minutes, and call Sonos Support to discuss it. Don’t post the resulting diagnostic number here, they get sensitive about GDPR.

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.

When you speak directly to the Support staff, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.

Okay, I’m doing that now. Thanks

Remember, as discussed in the Diagnostics - How do they work? thread, Sonos won’t look at the diagnostic until you call in. Don’t assume that you submit, and they contact you, it doesn’t work that way. I assume too many false positives, but I don’t really know.

FYI, as I thought about the issue more, the issue was not with SONOS or the Remote Control. The TV speakers cannot be turned off. They will not work when an external device is attached. SONOS Arc via ARC HDMI, for example. The more I thought about this, the more I thought that the issue was with the TV.

So, I went back to the original suggestion, and took it one step further. I disconnected the HDMI from the TV rather than from the ARC. I thought this might force the TV to reset to the external device, and shut down the TV speakers. It worked!!

Part of HDMI-ARC is that there is a single point of audio. That can be external, or internal, but as Sonos has implemented the ARC standards, any HDMI connection between a Sonos device and the ARC port on a TV will turn off the internal speakers. Optical connections, being one way, not two, don’t work in this fashion.