Is it possible for Google Calendar notifications alerts to sound off on my Sonos?

I don’t think you can. To my knowledge, those alerts appear on the normal speakers associated with the device running Google Calendar.

I suppose you could define the normal OS’s speakers via Bluetooth or AirPlay 2, but I don’t recall anything defining a separate speaker to play alerts on inside the Google Calendar app. 

It seems this used to be possible, looking at 

Google moving to Gemini (and leaving behind Assistant and Sonos) might have made this impossible again.