When i try to cast to sonos in the plexpass app the inly option available is android. No me tion of Sonos or any button to add a new device.
If i log in to the Plex android app and click the cast button i can see sonos there but it says i must get a plex pass to connect to sonos. Clearly i have a plexpass as i am using the plexamp app.
OK then it should be linked. just need to figure out why the android app does not think you have a Plex Pass. If you have already tried things like force quitting and relaunching app then would need to see the android logs
(I don’t believe Plex Amp supports SONOS casting)