Perhaps try removing the Sonos Skill/Action in your Google Home, give it a few moments to ensure all your Sonos products get removed from the App.

Next power-cycle your router and after the network settles, just ensure all your speakers reappear in the Sonos App. Then check for any further Sonos updates and install those, if necessary. 

Return to the Google Home App and reinstall the Sonos Skill Action and ensure all your speakers are correctly placed into your Google rooms. 

Then hopefully all will then be sorted.

Hi Ken. Thank you for this. I followed your instructions and while it seems to have helped restore all my 6 speakers within the Sonos app. It is only allowing me to add Voice services to my Sonos move, not the beam or the Sonos one. It connects the Sonos move really easily but says it can't find any other devices. Music will play fine on the devices. They are all connected to the same network. 

A couple of things… when you add/added the Google Assistant to your Sonos devices were you using the same Google account and credentials as those in your Google Home/Assistant Apps on your local device and did the Google Home/Assistant App open on your device to search and find the speaker that you were installing the assistant on?

Secondly, you mention that the speakers and your mobile controller are all on the same LAN but what router and mobile device are you using? Perhaps ensure the WiFi bands are not isolated from your LAN. If you continue to have issues, maybe go onto see if they devices will setup correctly when you have just one Sonos device wired to your main router.