That’s really handy thanks. 
I found This which seems to fit the bill. 

any idea if this will automatically takeover the beam audio, or whether I will need to select it each time I want to watch my projector (like the line-in on the Play5) 

thanks

I use an Apple TV 4K with a 4K HDR 10 Monitor and a Sonos Beam.  No TV.

I use a $20 audio extractor ( AV access 4KCVH2H) with an SPDIF output and then the optional Sonos optical to HDMI adapter.

The whole thing works fine including the volume control on the 6 button Apple TV remote.

In Apple TV settings set remote to TV via IR and them in the Sonos app configure using the apple remote volume buttons.

I can confirm that the splitter and beam work well together - when Chrome kicks in it automatically takes over the Beam so it doesn’t require any selecting within the Sonos App. 
 

The power-in line on my splitter was a bit dodgy so meant the chrome kept loosing connection, but since I’ve stabilised it everything runs fine. 
 

Extra tip… the Beam will learn the remote control for your tv/projector and you can use it to control the volume - on my Optoma it wasn’t possible to deselect the built-in speakers - so what I did was plug a 3.5mm headphone jack into the output socket and this cancelled them out without effecting the audio to the Sonos.