Must be something your end - it’s working fine here with the iPad or iPhone volume buttons when airplaying to a Sonos speaker or group. I used the Apple Music App too.

The above was showing iPad - attached is my iPhone Airplaying and adjusting volume with its buttons. Volume is changing on the Sonos speaker.

Same here, works OK from iPhone and iPad with Apple Music and Apple Music Classical.

Thanks for your answer! I tried everything this morning without success, but now, after I restarted my devices it did work!

I had the same problem today. Restarting the Sonos didn’t help. Had to restart my iPhone to get everything working again

I’m having the same issue, phone running on iOS 17.6.1, app up to date. Restarted Sonos and phone. But when changing volume with the phone buttons it is changing the ringer volume on the phone. Any hold with this one please? 

The audio must be playing ‘through’ the mobile device for the phones hardware buttons to work. This is an Apple restriction/requirement - so if you AirPlay the audio to a Sonos speaker the mobile hardware volume buttons will then work.