@Sbuyle

Sonos speakers just are airplay audio receivers. If you want to play video content via airplay you need an AppleTV device as an receiver. 
Apples ios in many cases (e. g. photos app) doesn’t support splitting video and audio to send the audio via airplay. In the past imo there were a few apps on ios that can do so. At the moment with Amazon prime it works for me. 

As suggested, it is up to the app on the Apple device as to whether or not it sends an AirPlay 2 signal. Not all applications do. There isn’t much Sonos can do about that, unfortunately. 

@Schlumpf @Airgetlam thanks for your replies. I did figure out how to do it. You have to save the video to your files, then you can airplay it through the Sonos speakers.