Turn the TV and Sonos off at the wall. Unplug the HDMI cable from the two devices and reverse it. Then turn the Sonos back on at the wall, followed by the TV. See if that helps. 

(Presumably too the HDMI cable is plugged into the TV’s ARC or eARC HDMI port? And you have got HDMI Control or HDMI CEC set to “on” in the TV’s settings menu?)

Thanks for that suggestion. I will check what cables are plugged into which ports. This only happens occasionally, though. Most of the time, it works fine, and I get TV sound right away.

It always happens when you least want it to. . The power cycle seems to get everything back working again, but it takes 10 minutes for the cable box to reset. 

Is there a pattern to when it happens? Just for example, and consider what applies to your set-up: when your set-top box is already on when you switch the TV on, does sound play? Does it make a difference if you are tuned to certain channels when the TV comes on? Does sound only kick in if everything plugged into the TV is off? etc. 

It might help identify the culprit in the chain. BUT try what I have suggested earlier first...

I can’t say as I notice a pattern, or the other situations you mentioned. I think the last time it happened may have been after I had re-booted my router the evening prior. I’m wondering if it failed to re-connect to that. But other times, it has happened when I know my router had been up for months at a time. it’s very random, it seems. 

I’ll try a few experiments with having the Cable box on or off when I turn the TV on and see if that has any effect on things. Thanks so much for your suggestions!!

(And reverse the HDMI cable, and check the ARC/eARC HDMI port is used, and check HDMI Control is set to “on” on the TV)!