Wanted to post this up somewhere for people that might be tearing their hair out like I was without realising the issue.
TLDR: when going through Sonos app to enable Google Assistant for your Sonos devices (Sonos One, Beam or Move), make sure your pi-hole is DISABLED / your DNS setup is unfiltered and ok. Turns out my pi-hole was blocking something from resolving somewhere which meant Google Assistant could not find any of the Sonos devices (No Devices Found error in the final steps in Google Assistant app). This was even despite the Google Home app showing the Sonos account as “linked” and showing the devices in the “Rooms” shown - it just wouldn’t complete the final steps of actually enabling the assistant on them...
I probably shouldn’t be allowed to run my home network given such a simple thing, but sometimes the obvious/simple things are what you overlook, so maybe someone else will find this useful.
Long Story: I was recently trying to add a Google Assistant to my new Sonos Move, but every time I went through the Google Assistant linking process from the Sonos App, when I got re-directed to the Google Assistant app I would be hit with the “No Devices Found” message.
Tried lots of things; enabling various settings in my Ubiquiti Unifi setup (IGMP snooping, Multicast Enhancement, etc - https://help.ui.com/hc/en-us/articles/360001004034-UniFi-Best-Practices-for-Managing-Chromecast-Google-Home-on-UniFi-Network), removing and re-adding all devices from Sonos and Google Assistant, re-installing Google Home and Google Assistant Apps, unlinking and re-linking Sonos in Google Home… nothing worked.
Then I suddenly realised, since first setting up my Sonos and Google Assistant, I had deployed a pi-hole into the network that could potentially be interfering with some of the many domains that Google/Sonos might use for registering devices in the background. Went into pi-hole, temporarily disabled it from doing it’s thing so my DNS lookups would be totally unfiltered / unblocked - hey presto, Sonos devices all found and Google Assistant enabled on them one-by-one (as I had previously removed them all in my attempts to resolve the issue).