In previous versions of the Sonos app, the Sonos Systems Settings for the Arc, used to show the audio in settings (Stereo, Dolby Digital 5.1, etc).
In the new app this does not display.
Additionally, I occasionally get no audio out when I turn on the TV using my Apple TV remote. This also happens occasionally when I switch the streaming app on the Apple TV 4K from one service to another. To rectify the situation, I have to open settings in the Apple TV app, and change the temporary audio out settings to the Arc.