The Arc receives its volume commands thripough the ARC connection with the TV. I’d be checking a few things, it sounds like your TV isn’t receiving/transmitting the commands properly. The first thing I’d do is reset the atavistic, by unplugging it from the wall for two minutes. If that doesn’t change things, I’d reseat the HadMI cables at both ends, and reprogram / set up the Apple Remote again.

If none of that changes anything, I’d submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it.

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 phone folks, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.