Sonos Arc

That’s the way ARC is designed to work, it has nothing whatsoever to do with Sonos. If you want to use the TV’s speakers instead of an ARC device, you need to turn off HDMI CEC, which carries the ARC signal back and forth. Then the TV won’t ‘see’ any connected speaker, and will allow (force) the use of the internal speakers. 

I was in the Richer Sounds shop today and the one there worked properly, same set up.

Same issue for me with Sony A9G, I turned Audio Output->Speakers→ TV Speakers.  It switches for a moment and then reverts back to External speakers.  I tried turning eARC mode to Off just for the TV and it’s still external speaker. Not sure where HDMI CEC is or if it’s called something else on the Bravia. 

I have a Sony 8A similar model to yours, the tv in the shop was a different model, it may be an issue with these models, if you find solution please let me know.

@Airgetlam, ideas on what else CEC might be called?  The Sony site doesn’t give much in terms of the setup of this part specifically beyond what we’ve tried.  I can also ask other Sony/Sonos users in their forum.

It’s been years since I worked for Sony, let me go look at a manual.

I had the Sony help people remotely go through the settings on my tv, they found nothing wrong and referred me to Sonos.

Looks like the call it what it is, eARC. Oh, and BRAVIA Sync.

See https://www.sony.com/electronics/support/televisions-projectors-oled-tvs-android-/xbr-65a9g/articles/00137387

eARC is an upgrade from ARC it can handle more bandwidth, it is needed to handle Dolby Atmos.

No, standard HDMI ARC can handle the bandwidth necessary for Dolby Digital Plus, which can also carry Atmos. eARC is great, but not a requirement for Atmos.