Apple TV Temporary Audio Output Airplay
I found a solution to make Airplay speakers permanent instead of just temporary. It worked for my Sonos Airplay 2 setup. Now I don't have to constantly switch the audio output from 'TV Speakers' to my Sonos setup.
See this youtube video:
https://www.youtube.com/watch?v=wb4-U0TUB0Y
Hope that helps!
Cheers
Apple TV 4K (3rd generation)