You are actually getting Atmos from Netflix and Disney plus apps? I have a similar setup with an LG tv with e-ARC and havent gotten Atmos out of my Apple TV at all. Disney plus movies play in silence, as does Netflix Atmos content. iTunes movies also play in 5.1

This is the way that Apple encodes Atmos on their own content. Using MAT, which is part of TrueHD, which requires an eARC connection due to bandwidth considerations. You need to either petition Apple to change that, or get a TV with eARC to play Atmos on Apple’s Movies content. 

That’s the thing I have e-arc so it should work

Hmmm. Missed that, apologies. I’d be looking at the output settings on the Apple TV to ensure that it’s set up properly, and then the settings on the TV to ensure that it’s also set up to pass through the signal.

Unfortunately, I’m currently stuck with a TV that won’t even pass Dolby Digital Plus from an HDMI input, so I’m not much help. 

2 things to check: 

1. On your Apple TV 4K, go to Settings > Video and Audio and select Audio Format. Under Immersive Audio, check that Dolby Atmos is on. If you don't see Immersive Audio and Dolby Atmos as an option, you might need to change how your home entertainment system is set up.
2. Check your tv’s settings and under the audio menu switch from Auto to passthrough while you are playing Atmos content

Lastly, go to your Sonos app and check System Settings-About my system and check the audio output. 

Those are the exact options I have checked. If I put it in auto I get sound but no Atmos. In pass thru it is silence. 

Yes so this is what is so weird. I have a TV with eArc. I have Atmos on in ATV. I have the TV set to ‘passthrough’. It works for Disney Plus and Netflix app on the ATV, just not any movies from iTunes (even though the movie says Atmos!) So frustrating. I can’ t figure out where it is breaking down, just for iTunes content.

What really frustrating is some of it is working for you and none for me even though we have similar setups. I wonder if this has to do with the LPCM thing?

Also found this in another forum:

LG has released a firmware upgrade for the B9/C9/E9 supporting multichannel PCM passthrough.  This is great except Arc doesn’t support it yet.

Because of this new update, my Apple TV decides it should use LPCM instead of Dolby Digital 5.1 with Atmos enabled resulting in only 2.0 audio for non Atmos content.  The current only workaround is to disable Atmos and enable it when you want to watch an Atmos file.  Very frustrating as the previous firmware had no issue.

After calling apple, try going to settings, apps, then making sure quick start is on. See if that fixes Atmos issue for iTunes movies. 

That did it! Thanks!