The cheap Visio soundbar I used to have did not have HDMI-ARC tech, it was just an HDMI pass through.  The TV was not aware of any connected soundbar, and could therefore play at a separate volume.  The soundbar could not play audio from smartapps on the TV though.

We keep going in circles.  If Sennheiser (AMBEO Soundbar Plus) can do it (notwithstanding Samsung HW-Q990D, albeit it was Samsung paired with Samsung), then Sonos should be able to.  Again, there is no normal proper scenario where a soundbar should be stealing the ability to switch to whatever output the user prefers on their TV, be it internal speakers or headphones!! selected in the TV menu.  If Sonos is incapable of accommodating this, they should have an off/on functionality to shut the soundbar down per user preference. 

Nobody is adding a TV to their soundbar.  It is the soundbar being added to the TV and the manufacturer of the soundbar should be doing whatever is necessary to provide working solutions for the TV’s people have.

​@dintis 

What is it exactly that you want? 

I have three (3) Arc Ultra’s each connected to an LG TV. I’m not having any issues with sound. If I want to disengage the Arc Ultra I can do so by opening the LG sound menu (with the remote) and selecting internal speakers. However, that would defeat the purpose of paying $999 for an Arc Ultra to then disengage it to hear the audio through the LG internal speakers.

BTW, if one doesn’t want to use the remote to return to internal speakers just pull the HDMI cable from the Sonos soundbar and the sound automatically reverts to internal TV speakers. 

Furthermore, if you want a set of headphones to cut the audio to any soundbar; that feature has to be programed into the TV by the manufacturer. 

Again, Sonos nor any soundbar implements CEC that is handled by the TV. 

Perhaps you failed to read the original issue.  The issue is that at least on some TVs across all the manufacturers, the TV fails to successfully snd permanently switch to any sound outputs from own menu (as intended and designed), because Sonos seems to reset the connection and reassign it to itself essentially stealing it after a few seconds.  I want decide what sound output to use.  Not Sonos.  If I want to use the headphones connected to my TV,  I should be able to do it.  
 

No pulling of hdmi cable, pulling the power cable and other ridiculous solutions should be necessary.  Either implement a software fix/workaround or add a soft switch in the app (other soundbars can be turned off using the remote or built in power switch) to power down the bar or disable it, if the software fix cannot be implemented.

All I can say is that it must be your TV. On any of my three (3) LG’s with Arc Ultra I can open the sound menu and switch to Optical or internal speakers. Granted I don’t have an optical device connected but when I do select optical-out the audio disengages from the Arc Ultra.

 The soundbar can’t steal the the audio. The soundbar can regularly ping the TV to let it know that it’s there and ready to receive audio, and I suspect many soundbars do not do this.  It’s ultimately up to the TVs firmware to decide where to send audio.  If the TV wants to decide to send audio because the Sonos soundbar, or whatever soundbar, just pinged and let it know it was ready, regardless of where the user said it wanted to send audio.

That said, I don’t know for sure what the CEC recommendation is on this.  It may be that TVs ARE supposed to transfer audio whenever they are pinged by a soundbar, and maybe soundbar’s are only suppossed to ping during setup or otherwise triggered by the user.  From what I’ve seen CEC ‘rules’ tend to be rather loosely implemented with the manufacturers choosing what they think should happen, often in conflict with other connected device, rather than a well defined policy that gets them CEC certification.

It seems the Sonos way of using CEC (maybe “pinging” more often than other manufacturers do) is allowed by the CEC guidelines. Sonos might be of the opinion that this feature is beneficial to most users, that buy an expensive soundbar and want to always use it - remember Sonos sells mostly to affluent but not always tech savvy customers. Your use case of buying an expensive soundbar and still wanting to use the cheap built in speakers looks like a minority use case, though it has been mentioned on the community especially for users that would like to use a head phone (for which the Sonos Ace now is an option).

Your only option would be to temporarily turn off CEC (called Anynet+ on Samsung TV’s).