The CEC spec, last time I looked at it, doesn’t allow that. You could conceivably use the optical output, so that the TV was unaware of a connected Amp, but there would always be a slight possibility that the TV’s speaker are automatically turned off when the optical output is active, not to mention the opportunity for there to be a slight delay between the two sets of speakers.

I also failed to mention the challenges of what each speaker carries. Are the TV’s speakers supposed to be the center channel, and the Sonos the right and the left front speakers?

Your customer wants something that to my knowledge, can’t be done. With anyone’s equipment, not just Sonos’.

Why exactly do they want to use the (very probably crappy) TV speakers? I agree with Bruce, this is unobtainium.

Would the in-wall speakers be used as surrounds (which isn’t possible for an HDMI-connected Amp, by the way)? Then the client probably hates the look of a soundbar. A solution could be to use an Amp for fronts too. These speakers could also be in-wall. I do not know if it is advisable to use in-wall speakers as fronts though…..

Your client is asking for a) Terrible sound, and b) The impossible.

And a probably unbearable echo, due to the timing difference between the two signal paths.