In all solved cases of this issue, it has been an external device controlling the Sonos, whether a remote control under the cushions, or a stuck button on a controller device, like a phone.

Thank you very much. It does seem like a mechanical problem but we’ve gone through everything and we are still stuck. Thinking cap on.

Well we’ve done a restart everywhere and it’s working now, so...off and on (the old trick)

If it happens again, immediately gather a diagnostic and share the code with Support. So they should then be able to tell you how the volume went mad.

I also have this problem on my Sonos Arc which arrived yesterday.  Volume keeps maxing out to 100 and then carries on trying to increase but just results in horrible sounding beep beep beep because it can’t go any higher.  It’s as if someone is secretly trying to increase the volume.  I had to turn off touch controls in the iOS app.  I think either the app or the touch controls are broken.  Just wondering whether to return it back to Currys or not.

I think controlav’s suggestion applies equally to you as it does the OP.