Hi @Blue cat,
As far as I can tell, Sonos Radio isn’t available as a default service with Google Assistant (GA). You can still ask GA to play terrestrial radio stations through Sonos Radio (not Sonos Genre or Curated stations), but you’ll still need to specify ‘on Sonos Radio’ in your command.
Our support article for using Google Assistant on Sonos has the following:
To see available music services, open the Assistant app for iOS or Android, select your profile icon to get to your account page, and then select Music. Available services are listed under More Music Services. Music service availability will vary by region.
You can ask your Google Assistant to play music from iHeartRadio, TIDAL, and TuneIn Radio if those services are already added to your Sonos system, but they can’t be directly added to your Google Assistant app.
There are a few services that aren’t supported as default services on GA but can still be used by it on Sonos, and Sonos Radio seems to be one of them. I can forward this to the Sonos Radio team as a feature request for their consideration, though I personally don’t expect Sonos Radio to become available as a default service in the GA app.
I hope this information helps.