I've raised a bug with Apple already, but I was wondering if anyone else is experiencing this weird issue:
When playing music on the Apple TV and associated HomePod(s) in Home Cinema mode with ARC enabled, I am unable to add a Sonos speaker as the AirPlay 2 target.
As a workaround I can first add a second Apple device (be it an Apple TV or solo HomePod not running Home Cinema audio), then add the Sonos speaker into the group, and the audio plays. Interestingly enough, removing the second Apple device from the group does not stop the Sonos speaker from playing from the original Apple TV and HomePod combo.
I've been able to replicate this in tvOS 14.5.1 and 14.6. I'm assuming that it is likely an issue with the Home Cinema mode with ARC, which is in Beta after all.