Perhaps this might help?

https://support.sonos.com/s/article/3444?language=en_US

Many thanks. Much appreciated. 

I had read this already, but I get stuck on step 3 

“Before we add the Sonos home theater speaker, you will first need to complete the remote set up for your cable, satellite, or Blu-ray player. Select New Device and choose the device you’re adding”

I don’t have any cable. satellite or Blu Ray player. All is seamless theoretically. However, there’s a list for Home Theater systems, and Sonos is not included. 

Again, many thanks.

I only have the TV and the Sonos ARC connected with optical cable

I am having the same issue. ARC VIA HDMI worked until a recent Sonos update. I had to switch to optical and I can’t get it working. Very frustrating. 

General - external device manager - input signal plus - turn on for the hdmi arc port (hdmi 4 in MY case). Volume works fine but both CEC AAAAND input signal plus on the relevant port have to be enabled.