Just last month, Sonos sued Google for allegedly infringing upon its patents. The two companies had previously worked together to bring Assistant features to select Sonos speakers. Surprisingly, it seems that this partnership is continuing, at least for now, despite the ongoing litigation. First spotted a few months ago by users on the r/Sonos subreddit, the ability to set Sonos speakers as an Assistant-powered product's default device for music playback now appears to be widely available.
Users have been able to set a default speaker or group of speakers for music playback when making requests from a given Google Assistant device in the Google Home app since late 2017. However, when Google Assistant functionality first came to Sonos speakers earlier that same year, this particular setting was notably absent. There do appear to be some remaining limitations to this newest implementation, though, as a few commenters pointed out that Sonos products can't join speaker groups containing non-Sonos hardware.
To link your Sonos speakers with a particular Assistant device, navigate to the corresponding device settings page in the Google Home app (pictured above) and select your preferred speaker within the 'Listen on' section of the 'Default media playback' menu.