Play what AppleTV currently plays at Airplay / Sonos speakers

Maybe it’s a stupid question, but I don’t get it done.

I have built-in speakers in my restrooms, each fired by a Sonos AMP. If the TV is playing, I want them to play the output of my AppleTV as soon as motion is detected so guests and us at least have sound when they miss a few moments. However, I can’t find a way to define the source to capture the current AppleTV output. The entity of the Apple TV media player does not work. Do you have any ideas?