The exact message it gives seems to be “Sorry your Spotify premium account isn't properly linked. To play from Spotify premium please set up the same account on both google assistant and the Sonos app.” → yet Google Assistant linked to my Spotify Premium. Sonos linked to 4 accounts (family plan that mine is part of)

I have the exact same issue, only I never got it to work to begin with... I have deleted everything to my knowledge and started all over. Not only same account but even same email-address on Google, Spotify Premium and Sonos... and yet, this message you mentioned above.

I have the same exact problem. Both the Sonos app & Google Assistant say they’re linked, but it gives me that same error message after it says it’s about to play music. Very frustrating. 

I found a solution that at least worked for me!!! Seems they don't have the exakt same problem, but removing and then re-configuring the assistant worked on my issue too.

https://www.reddit.com/r/sonos/comments/dmzuv4/comment/f5kkyi8

Finally!

I thought what you did in google assistant and google home was linked (it more or less being the same application). But if I understand you correctly you need to disconnect both individually and give that a shot?

Going to try that when I have a bit of time to re-add everything again.

Followup. Sadly for me this did not work. I cleared everything and readded. Again the same message unfortunately.

and… after removing everything and adding it again and nothing happening I gave it one last try and removed my spotify account from Sonos again and added it again. Keeping the name of my account the same as it’s actually on spotify and not renaming it (so it was Asgorath and not my real name - which makes more sense in a family setting :p). I tried it again and low and behold… it works again. After this I did rename the name Sonos displays to my own actual name again. That last bit is only because we use a family plan and have 4 Spotify accounts linked to Sonos. I don’t think it actually mattered.

Dumb thing is, I did this before and reading others everybody has tried this. It more seems a matter of just keep trying until it works. In my case removing and adding my spotify account last in the Sonos app after everything else was done seemed to do the trick.

Perhaps try removing the Sonos Skill/Action/Device too in the Google Home App and wait for the Sonos devices to delete from your Home/rooms and then add the Sonos service again and put the speakers back in their correct rooms (if necessary) and see if that fixes it for you.

The trick for me seems to have been after everything I did a last ditch, remove and readd my spotify account (not just reauthenticate) again and now.. it works.

Now I can fully enjoy the Dutch GA messing up the mixture of Dutch with English music names again misinterpreting things. “Hey Google, speel folk muziek af” (Hey google, play folk music) - “Okay I will play pop music”. Progress!

I had the exact same issue, with a number of family accounts for Spotify. Removed Spotify account from Sonos and added back in. Now working.