I have 2 Sonos Arc soundbars. I have an LG C7 and an LG C9. On the Arc attached to the C9 the HDMI CEC controls do not work. I can get audio through the eARC. The Arc attached to the C7 works normally. When I move the C9’s soundbar to the C7 I can't get any audio at all and the CEC controls still don't work. The C7 doesn't have eARC. The C7’s soundbar moved to the C9 works normally.
Sounds like a bad soundbar maybe? On the Sonos app it claims that the C9 ARC mode isn't on but I have a Sony soundbar that I replaced with the Sonos and the ARC works. Also works on my PS5 and Nvidia Shield.