I’m having a very similar problem. I can add the Plex account to my Sonos but after doing so I’m unable to browse music.
When you say that you can cast from PlexAmp to Sonos that’s via the Sonos integration and not via something like AirPlay correct?
Does your Sonos account show as linked here? https://plex.tv/users/other-services
I’ve written up my findings for my issue here: Internal Server Error When Linking Sonos
It would be really great if we could get some attention on these issues.
