Had this issue with a C3 and Apple TV latest gen. The fix for me was to make sure another device which was on was properly switched off (in this case a Sky box). If that was on, the ATV is silent. Bizarre glitch 

Same issue also here.

LG C3 with Apple TV 3rd gen, Fire Cube 3rd gen and Sonos Arc with Sub and Era 300s.

Connecting the Apple TV alone without the Fire Tv, and the 5.1 audio sources are playing fine.

As soon as I connect the Fire Tv, no audio anymore from the Apple TV. Only Atmos and stereo sources are working fine.

The only way to get the Apple TV 3rd gen to work properly is to shut-down the Fire TV.

Tried to reset everything. All HDMI cables certified 2.1. Tried also different of them.

Disconnected everything, power cycles on all the devices, waiting 30 minutes, reconnected: same problem.

Passthrough, Auto, tried all the audio settings from the C3.

Not really understanding why it’s not properly working.
 

Sounds like the Fire TV Cube is interrupting the normal discovery process for CEC, which is managed by your TV set. If you can, turn off CEC on your Fire TV Cube, or insert a CEC blocker in between it and the TV set, something like this. 

Unfortunately I tried already both of them:

• Disabling the CEC from the devices settings in the Fire Cube, Restarting then both the tv and the Fire Cube. Nothing.
• Bought the CEC blocker adapter. Same outcome.

You may want to contact Amazon, then, since based on your experimentation, the issue is with the Fire TV Cube, and not any of your other equipment. 

Maybe try swapping their connection to the LG TV and have the Fire TV connected to the current Apple TV HDMI input and vice versa - set things up again in the WebOS ‘Device Connector’, or whatever LG calls that these days and perhaps see what happens in that case.

I tried also that. Still no success

Apple TV 4K 3rd generation, FIOS TV One STB, and LG C3 I lose sound. 

If I connect an Apple TV 4K 2nd generation everything works fine. Makes no sense, both Apple TV 4K’s on tvOS 17.

CEC off on all devices, using Harmony Remote

Haven’t seen a way to completely power down the FIOS STB, only goes to standby.,

An update.  About two weeks ago I got a response from LG.  At first they were going to set up a home service appointment.  They cancelled that and said I should report it as a software problem and gave me a generic link to where people report a wishlist for future features.  No help at all.  I mentioned Apple support thought the problem was with the Sonos Arc.  Last Thursday Sonos support spent about 2 hours on the phone collecting log files in different situations and sent them to one of their engineers.  This is the response I got from Sonos:

 “... thank you for all the information provided. 

Furthermore, I have spoken with one of our engineers and he mentioned there were no reports about this issue until we got yours. Then, checking on all the symptoms, we concluded that it could be related to the incoming CEC or the board on the TV itself not working properly.  

I understand this can be an answer that you did not expect, however, it seems to be out of our settings since the Arc is reading the information from the device to which it is connected. In this case, I can recommend some other steps that may help you with this matter: 

• Try different HDMI ports on the TV to connect Apple TV.
• Check different audio formats on the TV itself, for example, PCM.
• Also, check different audio formats besides Atmos on the Apple TV. 

If you have any issues with scheduling or other questions, feel free to reply to this email or give us a call at 1-800-680-2345.”

So now I am back to using my Apple TV 4K 2nd generation and waiting for some sort of update from either Sonos, LG, or Apple or maybe a 4th generation Apple TV.

I really like the way Apple TV works as far as navigation and features so the 2nd generation is the best option for me.  There probably is something with how CEC works with all of the device since having only the Apple TV and Sonos Arc connected works.  At least on my set up if eARC is enabled I can switch the CEC off on the LG but it immediately switches back on.

Latest update.  LG sent a local technician out.  He said there were no error codes showing on the LG TV when I was having the sound problem.  I can reproduce the problem.  With only the Apple TV and the Sonos Arc attached the Apple TV 3rd generation works fine.  Adding a third device causes the problem and unplugging the third device brings the sound back.  The technician didn't recommend replacing a circuit board since he couldn't find a problem.  He recommended I trade in the Sonos Arc and get another brand of soundbar.   I am back using the Apple TV 2nd generation and I can have all of the HDMI ports connected.  I still have 2 weeks to return the LG TV to Costco.  I considered switching to a Sony but I really like the LG TV except for this problem and I am not sure that switching the TV would solve the issue.