Does this happen when you play music from a streaming service with the TV off?  Most likely, a volume up command is coming from the TV or from your app, and this should help determine which it it.  I’ve also seen cases where the buttons on a TV remote get stuck.  I would look at that and maybe take batteries out of the remote.  If you determine that the TV is not the issue, maybe shut down the Sonos app on your phone, or restart the phone.

You can also submit a diagnostic  and contact Sonos so they can tell you exactly where the volume command is coming from or the unlikely chance that there’s a hardware issue.

Is something resting on and pressing the tv remote volume button?

There was indeed a thread a couple of days ago where someone had to ‘clean’ their Arc, as apparently there was some dust affecting the capacitive button on the top of the Arc. 

Neither of these seem likely for this situation, but there has also been cases where someone else in the household thought using a different phone to turn up the volume would be a good prank (I may or may not have done something similar in the past).   Then there have been cats.

Thanks Danny and others. I’ve isolated the problem to the tv remote. Volume from phone control for radio or Spotify streaming is normal. TV remote (Apple) is also not holding on mute, I’ve discovered. Must be the remote itself (it’s been charged overnight) or the Apple TV that the signal comes through.

I’m calling in store help.