Are you talking about the privacy settings on the Apple TV?  Which setting, do you switch it off and then back on?

LG settings

Support; Privacy & Terms; User Agreements 

Thanks.  That worked.  Not sure that I will use it but interesting.

It’s the only way currently to maintain the proper settings on the LG for best sound. 

I think I have solved the problem.  These are my current settings:

Apple TV 4K 3rd generation, 128 GB, ethernet

    Video and Audio
        Format 4K SDR
        HDMI Output YCbCr
        Chroma 4:4:4
        Match Content
            Match Dynamic Range On
            Match Frame Rate On
            Quick Media Switching On
        Audio Output Receiver Speakers
        Audio Format
            Change Format Off
            Dolby Atmos On
        Audio Mode Auto
        Color Balance Balanced (with Enable Dolby Vision not selected, I 
            used the    color balance calibration with my iPhone.  With Dolby Vision enabled this is not an    option.)

LG 65OLEDC3AUA

    General
        External Devices
            HDMI Settings
                4:4:4 Pass Through On
                Quick Media Switching On
                HDMI Deep Color 4K
                SIMPLINK (HDMI-CEC) On
        Sound
            Sound Out HDMI(ARC) Device
            Advanced Settings
                Select HDMI Input Audio Format  Bitstream
                Digital Sound Output Pass Through
                eARC Support On

I am not sure why this works or if these setting affect other areas with video or audio.  Let me know what you think.

I thought everything was OK but it wasn’t.  I can switch HDMI inputs without problem but if I switch to one of the WebOS apps like LG Channels the problem recurs.  If I unplug the TV and plug it in again the problem resolves until the next time I use WebOs apps.  

Looks like you turned off Dolby Vision as a format well. 

From what I understand choosing 4K SDR only applies to the home screen.  So long as match content and match frame rate are on, switching  to Dolby Atmos content puts the Apple TV in Dolby  Atmos.  That seems to be my experience.  

I am not sure why I have problems using the WebOs apps.

I don’t use LG apps ever. The apps on the Apple TV are far superior to my experience. 

I am having the same issues again so I do not have a solution.  I am back to using my Apple TV 4K 2nd generation.

Try disabling 4:4:4 pass through, that setting shouldn’t be used for the Apple TV. It’s for PC/Consoles.

like I said I have zero issues on both a C9 and G3 so no idea why you’re having so many problems.

When I had everything working I tried disabling the 4:4:4 pass through just to see if I could and the problem recurred.  At least with those settings I could get it to work for a while.

After six months… LG is finally refunding me the entire purchase price of my G2. 

Wow.  I am not sure a different TV would solve my problem.  All of my devices work fine except for the Apple TV 4K 3rd generation.  I think the problem lies there.  It’s good to know LG would provide that level of support.  Thanks for the reply.

I have the same exact problem as you. The problem lies between the LG, the Apple TV, the Sonos Arc.  I will likely attempt buying the G3. If that doesn’t work, I’ll return it and buy the Sony A95L. 

If that fails… i’ll dump the Sonos Arc and start fresh.  Life is too short for this kinda trouble!

by the way, and just to be clear, LG support service was the worst I’ve ever experienced. Absolutely horrible. It took six months to find a resolution with their support department. The only reason I was successful is I was willing to spend hours and months pressuring them for a resolution. I’m gnarly like that, beyond what most healthy humans would tolerate :)

I updated the Apple TV 4K 3rd generation to tvOS 17.0 today.  That had no effect.  Back to using Apple TV 4K 2nd generation. 

Is there a way for an author of a question in Sonos Community to remove the “best answer” label?  Initially I thought the problem was solved but it turned out it wasn’t?

Also just a comment.  I would think the Sonos Arc, LG OLED C3 and Apple TV 4K 3rd generation are popular products.  I have only found one or two discussion about similar  problems that I am having with eARC and Dolby Atmos sound and I have searched many websites.  Am I one of the few that has this problem?  I could see many people having this particular combination of products not just me because of their popularity.  Seems weird.

Not for the author, no, it requires a forum moderator. However, it does seem that the vast majority, at least from what I can see, ignore that ‘function’ completely, likely for that very reason. 

This morning I tried unplugging the cable box and Roku from the LG HDMI and the Apple TV worked fine.  As soon as I plugged in another device the sound problems returned.  Do you think that is a problem with the Apple TV not working well with other devices or the LG TV not handling multiple devices or something with the Sonos?  They all work with the Sonos unplugged and using the TV speakers but eARC is turned off automatically.

There was a link to send an email to the CEO of Sonos so I sent an email.   I was hoping to get a higher level of Sonos support but so far no response.  Do you think they really answer those emails?

No, I think there is a device that isn’t honoring CEC properly, which is being reflected in the ARC/eARC output. 

The last person I’d contact is Sonos’ CEO. The amount of time that they have to help troubleshoot is likely less than zero, and I’d expect them to hand off my issue to someone else. I’d instead be trying to resolve my concerns through Customer Support. While there could be an untrained person there, the chances of them actually helping my situation are much, much higher than any CEO, who’s job it is to make strategic decisions, not help users with an issue. 

This thread has helped assure me that I am not insane. Running into the same/similar issues. 
LG TV + Sonos Arc + AppleTV 4k 3rd edition. 

I just upgraded to an LG C2 (OLED77C2PUA) TV this week (from a 7 year old LG TV). On the old system, CEC and Arc worked seamlessly, but depending on the audio encoding, I’d get horrible lip sync / audio lag issues. Could be 2 different shows on Netflix - one Atmos the other not, and it would require different audio delay configurations in the TV and/or Sonos Arc settings to compensate.  Annoying. 

Setup my new TV and everything was synced gloriously… until you switch inputs and suddenly audio through the sonos died. 

My current config which seems to work perfect regardless of input is: 
 

Sonos Arc (updated to latest software)

TV: LG C2 OLED77C2PUA (updated to latest software)

Philips Hue Sync box (updated to latest software)

Apple TV 4K (updated to latest OS)

• Everything connected using 8k HDMI cables

Apple TV » Hue Sync Box (HDMI port 4) » TV (HMDI port 3)

TV » Sonos Arc (HDMI eARC port 2) 

Also have a Switch & and a PS5. Switch connects to the Hue Sync Box, the PS5 is direct to TV. Adding/removing these seems to have no effect. 

∴ LG TV sound settings: 

• Output: HDMI Arc Device
• “eARC support” on: no sound from Sonos
• 🟢  “eARC support” off: Sonos works

adjusting these seemed to have no effect

• Simplink is turned on
• Passthrough: Auto

∴ AppleTV Settings

• Audio set to Auto (not 16 bit)

∴ Philips Hue Sync Box Settings

Messing with these doesn’t seem to have an effect

• CEC Power State Detection: On
• New Input Detected: On
• HDMI Device Turns On: On for all 4 ports. 

Given what others have said here, I might try to turn on eARC support but turn off Simplink on the TV and see if that preserves the audio codec flexibility of eARC provides. I’ll post back later today when I get a chance to test. 

After a bunch more experimentation, the only thing that consistently works is leaving off “eARC support”. 
I get more consistent connections that way, but occasionally following an input change or waking from sleep, the TV will reset to “TV Speakers” and I have to update it to HDMI, and then it takes some time for the Sonos to wake. 

Sonos shows Atmos and all the other audio formats regardless of “eARC Support” being on or off; so not really sure what that toggle is doing.  I had thought that “eARC support” enables Atmos, and with it off it would only be limited to Dolby 5.1 or some older audio encoding—but that’s definitely not the case.  But… the moment I enable it, sound from the Sonos Arc is immediately disabled. 

I encountered this same problem with Apple TV 4K 3rd generation and Sonos Arc. However after several attempts and a lot of web research I believe the problem it’s on the LG TV side  I realized that if I change hdmi cables or disconnect my tv from power the arc wouldn’t get any sound from it with or without Apple TV. I had to disconnect all hdmi cables from tv and Arc. And both systems from power for a few minutes. And then do the Arc tv setup in order to trigger LG to recognize a new HDMI connection. Not flawless beacause I’ve already had to factory reset the tv 2 times in order to the TV recognise a new connection from Arc. 
By the way. My LG Tv it’s the Oled C3 model.