Perhaps try powering everything off for a few minutes (not standby) - TV, soundbar and connected peripherals and see if that resolves the issue. If not then try switching off HDMI-CEC control on the Apple TV (reboot the Apple TV box) and see if that may solve the matter too.

Had tried a power cycle on all devices a few times already unfortunately - just tried turning off CEC and rebooting Apple TV but still having same issue. 

I have the LG C9 TV and Arc/Surrounds/Sub and it’s working okay here - maybe try toggling Simplink off/on in the TV settings - Do you have Sonos set as the manufacturer for the Arc in the device connector or whatever that feature is now called in WebOS? 

If the issue persists, then within 20 minutes of the audio-dropout issue occurring, submit a Sonos diagnostic report from the ‘Settings/Support’ area of the Sonos App. Make a note of the reference generated, and when practicable, contact/chat to Sonos Support Staff about the matter via this LINK and see what they can suggest to resolve the matter.

My LG TV’s HDMI-eARC port by the way has the digital sound out set to ‘Pass Through, so if you have yours perhaps set to ‘auto’ then maybe give that a try too.

Weirdly enough I’ve just been messing around with my eARC HDMI cable and in the process managed to discover that the audio is cutting out when its using my TV speakers so it’s likely an issue with the Apple TV instead of Sonos. Ah well, thanks for the help Ken!

Just while I’m speaking to someone with the same setup, I often miss the first few seconds of audio whenever I start a new YouTube video. I’ve assumed this is same for everyone as the audio adjusts, but is it something I can fix?

Hope you are able to sort the issue with the Apple TV.

Again, the missing audio at the start of playback is not something I’ve seen/noticed but is that when you start playing any/every audio source, or just a particular source?

Aha! Weirdly enough I’ve just been playing around with the sound settings on the C1.

On the ‘Select HDMI Input Audio Format’ - I had HDMI 1 (Apple TV) set to Bitstream. This was causing the second-or-two delay to audio starting, and after I’ve changed it to PCM, the audio doesn’t cut out later on in the video either.

The info suggests Bitstream is for the ‘latest surround sound technology’ so I assume it’s intended to have this turned on. Wonder why it’s causing issues for the TV on these particular videos? (And sound delay for all videos)

@NP90,

My older TV is currently set to bitstream, but if PCM sorts it for your setup, then that’s great.