Did you ever figure out this issue? I am experiencing the same thing. I have a Sonos Arc hooked up to an LG C1 TV via HDMI eARC, and am using Alexa as my voice assistant. When audio is coming from the TV, I hear the microphone turning itself on and off. It happens randomly, usually every few minutes though.

I have the same problem. Seems to be related to Google Assistant although when using Sonos Voice, you can hear Mic going on and off. Turned off Google and issue goes away.

Internet search suggests there are many with this problem!

I am going to try and disconnect it when I get home, but it seems weird that the other person in the community having the same issues also disconnected from the tv and moved it to a different room and still has the same issue.

There could be a fault in ARC, I might think, causing the mic to be switched on or off, which is why I was suggesting testing while the HDMI cable is disconnected, to see if it was reproducible. The challenge is that there are so few people reporting this as an issue, it certainly seems quite unusual, despite the fact that you are. Much easier to find, I’d think, if hundreds upon hundreds of people were submitting it as an issue, along with diagnostics.
 

I don’t expect minimum wage folks on the front line to be fully conversant in finding arcane circumstances in diagnostics, just the basics. Which is why I was suggesting a higher level, someone who may have more experience in reading diagnostics, and finding perhaps that one odd data point that doesn’t make sense.

Well if it’s unlikely to be a Sonos hardware/software fault, and the issue persists even when the Arc is just playing music with the tv not even on, then where would the issue lie? It has to be one or the other. It seems incredibly unlikely that both Arcs I received have the same issue. And it seems even more unlikely that the other person in this community bringing up the exact same issue, and who has also gotten a replacement, would have two broken arcs.

Shouldn’t this be easy to fix through the diagnostics? I assume they can see what’s triggering the mic.

Or perhaps it’s an issue with the signals it’s receiving from the TV via the ARC/eARC path? I’ve got two Arcs, and neither exhibit this behavior, and until recently, both were connected to Vizio TVs. 

I’d be tempted to first make sure the TV’s software was up to date, then reboot the TV by unplugging it from the wall for 5 minutes, just to eliminate that possibility. Then I’d be tempted to disconnect it from the TV connection completely, and see if it could be reproduced, which would be a good indicator of either a Sonos hardware or software fault. But given the low recurrence of this in the wider population, it seems unlikely.

Suspect you’ll need to get to at least level 2 support, it wouldn’t surprise me if the basic front line folks just don’t have the experience or training to understand this super rare issue. 

It does it whether the TV is on and playing sound or whether it’s independently playing music, so it can’t be the arc.

I’ve logged several diagnostics but it’s pointless. The support team is all in India and give basic, unhelpful advice along the lines of “did you restart your router.” I don’t understand how I could get two Arcs which have the same exact issue. Seems too coincidental. I’ve seen others having the same issue, including another active thread on here.

It just seems like the designed it poorly.

Have you submitted a diagnostic report and spoken to support? I don’t know if the diagnostic report will indicate where the “mic on” command came from but might be worth an ask of Sonos?
Could it be an hdmi-arc command? Does your tv support voice control? Just putting a few ideas out there for further suggestions/contributions to try to move forwards and resolve it. 

Haven't tried it but wouldn't I then have to remove it from the surround system? I also had an issue when I swapped the arcs where it wouldn't let me remove the surrounds from the previous room. Had to do hard resets of all devices. Such a pain.

I don't know why room would a difference. I tried it with no AC on. It's also about a foot from the tv. I tried to hold it up to the tv to see if that would trigger it. It did not. 

The chances that I got two broken devices seems slim but this is totally bogus. It's obviously poorly designed.