There are more topics about this, see for example

Maybe use Google to find more, since the search function on the forum seems to be of little use.

@abacslag

Do you use an iPhone from work with installed mdm software of an admin? That sometimes is reason for that. 
Also if the Sonos device is added to Apple Home there are problems with Airplay to Sonos. 

Thanks Ralf. It is private iphone. 
Csaba

And what about Apple Home? Go to iOS settings for Home and check if password for speakers is turned off. 
 

Also I would delete Sonos Speakers from Apple Home App.