Dolby Digital 5.1 / uncompressed 5.1 output as multichannel pcm 7.1

So after a year I finally got my sonos arc /Apple tv4k / lg cxoled tv to work properly. I reset my tv and Apple TV. I opened up Apple Music. Dolby Atmos worked no problem. Then I went to Netflix on my Apple TV. Dolby Atmos worked and also the 5.1 finally worked. Before this I always had issues with no sound or coming out as pcm7.1. So I checked my tv sound settings that are now reset to default factory. It has my hdmi set to bitstream. Digital sound out to Auto. EARC off. Now everything works fine. Crazy. I guess eARC and pass through of any combo failed. Hope this helps. 

You are getting Dolby Atmos audio from the Apple TV 4K with eARC turned off?

Yes. Earc off and set to auto not pass through. 

I didn’t know Dolby Atmos audio from the Apple TV 4K was possible without eARC. Are you sure you are getting real Dolby Atmos audio or is the TV just converting the audio to Atmos? Do you hear any height channel effects above you?

I can't say for certain. It does sound great so assume it is what is says. When I go to non-Atmos songs it shows as PCM on app. When I turn on earc, it sometimes works or goes to pcm 7.1. Also sounds good. The big issue is with Netflix on Apple TV. If I leave earc on, sometimes I have no sound at all. 
My understanding is you only need arc for Dolby atmos. Maybe for true Dolby atmos via a blue ray player you need earc. Not certain. 

ARC allows Dolby Digital Plus, which carries the ‘lossy” version of Dolby Atmos. eARC allows MAT/Dolby Atmos, which is the “non-lossy” version.

Worth reading up on the two versions of Dolby Atmos...my impression, from last year when I read up on it, it sounded more like “lossy” meant fewer speaker channels, not a lower quality of sound. And certainly in a Sonos system, you’re limited to how many channels can be listened to. 

At the end of the day, though, it’s still a sound bar. A rather good one, to be sure, but if you’re that concerned about disparate channels, you likely shouldn’t be considering a sound bar at all, but a fully featured amplifier with wired separate speakers. 

Hmm. Interesting. On my Lg tv smart apps using Netflix I get Dolby digital plus with 5.1 content and Dolby atmos with atmos content. That is with earc and pass through enabled. The issue to me seems to be Apple TV. Same settings with Apple TV and I get no sound with 5.1 content and atmos content will show as 7.1 multichannel pcm. Apple TV with earc off will give me 5.1 with 5.1 content and atmos with atmos content. Anyways, very happy with sound. Just had to find the right settings. 

Well first off streaming apps use the DD+ lossy version of atmos.

It’s all to do with the sound carried over HDMI, which it’s not when you stream from onboard TV apps. I find it strange you get no sound with 5.1 audio with passthrough and eARC enabled via apple tv device.

I have no issue with those settings, just that all DD 5.1 audio is output as multichannel 7.1.

Its all a bit of a minefield, there was a similar thread to this to do with audio and missing channels on the sonos arc, big problem is Sonos support absolutely sucks massively. They used to reply often but now they pick and choose who to reply to.

Thank you, this has solved my issue.