Ah, so not a Sonos thing, an issue with LG?

Not an LG thing IMHO. I have replaced LG motherboard and power supply thrice now.  it is either Sonos or Apple TV.

If it was either (or both) of those devices that you mention here, then wouldn’t @Airgetlam be having the same issue, regardless of the TV, particularly if the TV is set to audio pass-through?

The ATV can be in conflict with the LG, but not the Vizio.  if I switch out the LG TV 3 times, and the problem doesn’t go away I have to assume the problem is not the LG TV.  

I wish I had another Sonos Arc to plug-in. If the problem doesn’t persist with the 2nd Sonos Arc then it can be assumed my existing Sonos Arc is at fault. 

But… you mentioned earlier, that if you set the LG TV digital sound-out to ‘Auto’ on the HDMI-ARC/eARC port - that it fixed the audio issue from the Apple TV. So I can’t see the soundbar being faulty. That sounds much more-like the fault lies with the TV and actually, that’s the only difference between your setup and @Airgetlam’s… Maybe there is a fault in the LG firmware, or hardware, that conflicts with the Apple TV. Replacing the LG hardware is not likely going to fix it either, but maybe a future firmware update will sort it, perhaps ?

No offense, but someone else must have said that sound out to Auto worked for them. That is not so with me.

 However, it is the eARC that the Sonos is plugged into. If eARC is turned off, then it works fine business. It could be eARC implementation by Sonos.  It could be eARC implementation by LG.   It could be the audio out implementation of the ATV. 

Keep in mind it was the upgrade from the Apple TV second generation to the Apple TV third generation that caused the audio issue to begin.

Ah I thought that TV sound-out setting fixed things for you, my mistake then if it was switching off eARC on the TV.

Did you ever try swapping out the HDMI cables to see if that might make a difference? Or do you have a friend/family member with an Atmos capable TV that you could try things on, just to see if you can eliminate some things and perhaps discover where the problem(s) may lie?

Haha… all cables, components etc… have been switched out months ago. I have effectively switched out the LG TV three times as I have replaced the motherboard and power supply each time. Other than the display itself, the motherboard and power supply is essentially switching TVs, because there is nothing in TV other than that.  I do not have another Sonos system or television to swap out.  

Remind me to never buy a damn apple box to watch tv....

FYI

When I either, reset privacy settings or unplug the LG and plug it back in, all audio functions normally… (Until inputs have been switched on the LG: e.g. watch cable and then switch to ATV)

I agree.  Using LG Universal Control to switch HDMI input seems to be related.  Just curious if the person with a Vizio tv has eARC enabled?

If you’re talking about me, no, the Vizio is too old to have eARC. It does have ARC and that works just fine, in fact I tend to use the Apple Remote, instead of the Vizio remote, just because fewer remotes is better.

ATV works fine with ARC.  eARC does not work properly. 

same problem here. for now my solution is switching inputs. i go to tv and quickly switch back to the hdmi (earc) and ATV works until i turn off.  hope this temporarily helps.

I’ve used an Apple TV 4K 3rd gen with both an LG C9 and now a LG G3 and have no issues at all and eARC works perfectly with my Sonos Arc, no idea why you’re having issues. I’ve used both tvOS 16 and 17 also.

That didn’t work for me, but thanks for the suggestion.

I spent an hour on the phone today with Apple Support.  We couldn’t figure out why the Apple TV 4K 2nd generation works and the 3rd generation doesn’t.  I swapped out the HDMI cables and tried different HDMI inputs on the LG OLED C3.  I’ve already tried Sonos support.  I may try LG support but not too optimistic.

At a quick glance, there seems to be others reporting this on the Apple forum & Reddit - it doesn’t appear to be a Sonos issue, but LG TV’s are mentioned in each case.

https://discussions.apple.com/thread/252455967

https://discussions.apple.com/thread/250260832

https://discussions.apple.com/thread/254624095

https://www.reddit.com/r/appletv/comments/zivze4/apple_tv_4k_3rd_gen_lg_c2_lg_soundbar_has_trouble/

Those Apple discussions are either quite old or not apples to apples (no pun intended!) to this situation.

Some mention Apple TV 4K (3rd gen) the above links were just a sample I saw, so maybe it’s an ongoing issue and it seems to be related to LG TV models too. 

I spent an hour today talking to LG support.  No luck.  They had me unplug the TV and press the power button for 60 seconds.  When I plugged it in and restarted the TV the Apple TV worked as expected.  When I switched the HDMI input to another device and then back to Apple TV the problem recurred.  The Apple TV 4K 2nd gen works but to me there is a subtle difference with better sound and picture with the Apple TV 3rd generation.  I just have the sound issue when eARC is on.  For now I will use the built in apps in WebOS and try again if there is a software update from Sonos, Apple, or LG.

For me, the WebOS app experience is by far the weakest when compared to ATV 2nd gen.  I’m replacing the motherboard on the LG this weekend for one last time. If it doesn’t fix the issue… And I would be shocked if it does… LG has said that they would replace the TV. I will believe that when I see it though.

For now I use the ATV third GEN, and when I switch from cable to the ATV… I go into Settings and remove one of the privacy selections from the support section and that resets both the TV and the ATV 3rd gen, which allows it to function properly. It’s a pain in the ass but oh well. I will not settle for a lesser experience than ATV 3rd gen.

I know Sonos could help us to figure out this issue.  It’s unfortunate they don’t.