What type of Google Assistant device are you speaking to? Is it a Google Nest Mini etc; (with it’s default speaker set to a Sonos product) or an actual Sonos product with the Assistant built in?

Presumably you have the Sonos action/skill installed - try re-checking/scanning for new devices.

What paid-subscription music service are you using as default in your Google Home and is that also installed in the Sonos App with the same login credentials?

Thanks for the suggestions.

I'm using the assistant various Android phones, Tablets and iPad. The Sonos skill is enabled.

Spotify is my music service and the same login credentials are used on all devices.

As I said it all works with the exception of having to say which device/room I want to listen to the music on.

Unless you have a Google Nest ‘speaker’ or ‘display’ device with a Sonos speaker set as its default for music, or a Sonos device with GA installed on it, (like a Sonos One for example), then you will otherwise need to mention the room name to the assistant, particularly to GA on mobile devices as it would not know which speaker (if you have more than one) to play to, or whether to play on the mobile device itself.

That explains it perfectly - many thanks.