Best answer by Stuart_W
Have a TV with Youtube app installed and a Playbar. Also if you have a set top box with the YouTube app (I have a Human HDR with YouTube).
Use a Connect, Connect:Amp or Play 5 and use the line in from your YouTube source (probably PC or Mac). This likely to be rather clunky and will have an audio delay of around 70ms from picture to sound.
Generally though it is worth noting that Sonos is not designed to work as an output for YouTube or rather YouTube want you to watch the videos rather than just listen to the audio.