As a test, remove the network connection from the surrounds.

@Frustrated202020 

Just to clarify… though the soundbar is connected via lan the wifi module has to be activated for use as a surround setup. Is it?

Just one of the two rearspeaker being cabled imho isn’t a good idea. Have you tried using both of them wireless?

Thanks for your suggestion - This is how the Arc is configured - Is this correct? I will try disconnecting the rear surround from the network cable also.

I’ve now done this and am testing this out now and so far so good! Why does this cause a problem?

Your Arc is set up correctly…

I can’t tell you in detail, why you get the problem with a single rearspeaker cabled, but from my own experiences and feedback of other users a setup with both rears connected the same way works quite good. 
I suppose otherwise there could be sync issues because of the different ways of connection. 
It it works for you now, I would keep it this way. 

Really appreciate your help - thank you. Fingers crossed the problem is now resolved. Will report back if I experience it any more!

Like so many things Sonos “It just helps.” which of course sucks as an answer but since we users are locked out of seeing the internal data all we can do is experiment and share what has worked for us.

I fought blindly following the suggestions here many years ago when I was a new Sonos owner, preferring to do my research and see for myself just what the problems were and how the suggestions would fix them. I failed completely on two of the most common ones, static/reserved IP addresses and wiring my Subs and Surrounds. I eventually gave in and did/undid both and the problems stopped. I’m still not happy not knowing why but at least my Sonos work.

I’m speculating but it is likely that there are minor, intermittent communication delays for the current connection method and the player then attempts to be smart and use the other method, but there is another delay while working out the details. During these delays, the unit might flip-flop between methods, eventually running out of data, and has no choice but to mute.

Thanks for your suggestions as to what may be happening - makes sense.

As an update, yesterday I carried on watching the show I was watching on Saturday night (where I was experiencing the millisecond sound drops) and didn’t have a single sound glitch whatsoever. Seems to have done the trick. I hadn’t re-booted either the speakers or the TV… Literally just pulled the network cable out of the rear surround and hey presto - fixed. I have all my fingers crossed that this is now resolved. Thanks to everyone for their advice - I really appreciate it.