You will need to switch off HDMI-CEC to return to the TV speakers. It’s known as ‘Bravia Link’ or ‘Bravia Sync’ on the various Sony TV models.

Sony support on site and looked at the problem.

The problem does not arise with the soundbar you brought with you (not a Sonos) and the existing HDMI cable. The output can be properly controlled or changed via the television. Everything works as desired and as it should be.

When the Sonos ARC is reconnected, the problem occurs again immediately and audio controls no longer work.

I suspect the problem is on the part of Sonos :-(

I have now obtained such a CEC blocker, so that the control is so far established that the output can be switched between TV and ARC.

This is of course not satisfactory because I also had the opportunity to try out other soundbars. Result, with the exception of the ARC, none of the others produced the problem.

Hi

This may or may not make a difference. However, the manual for your Sony AG9 suggests that eARC be set to AUTO to allow the TV to properly recognize a capable CEC Audio Return device when connected to the HDMI eARC port (such as the Sonos Arc).  If you are trying to do some type of manual switching between the TV speakers and the Sonos Arc you most likely will experience connection issues. 

The Sonos Arc is a generic device relating to HDMI CEC eARC connectivity. Faults are always related to how the TV manufacturer chooses to implement CEC as there are no standard implementation rules.

Samsung, Sony, LG to name a few all implement CEC differently. This community (IMO) seems to address more CEC issues with Samsung and the least with LG. 

Question: If you feel the Sonos Arc is the issue versus other sound bars you tried then why not return it?

Hi, why don't I send the arc back? I have even more devices from sonos and of course I would like to take advantage of the benefits.
and I would not have thought that the arc is so stubborn.
all other soundbars from other manufacturers work better in this regard. (bose, samsung, ...)
it is disappointing that sonos does not work with Sony.

LG