I’ve read all the other forum posts related to this topic and couldn’t find a resolution. Sonos, please help us out here!
When I Airplay movies and shows from my 2020 iMac to my Apple TV to my Sonos speakers, it works great. I get picture and sound. Wonderful. 
When I try to use Airplay screen mirroring to do the same, I get picture, but no sound. It doesn’t matter what device I try (2013 MacBook, 2020 iMac, iPhones, iPad…), I get picture but no sound. 
I have hung onto my clunky old wired speakers for this reason. Why do I still have to use my old junk speakers half the time when I have my nice new expensive Sonos speakers? Please help!