Arc + ATV + Non Atmos Content

I’m guessing it’s a TV setting since you’ve got full eARC support (and congrats! you’re one of the lucky few). 

On your TV, go to Settings > Sound > Sound Out. There you should see settings for HDMI ARC. Make sure to set Digital Sound Out to Auto / Pass Through. If the Sonos Arc is connected to your TV via the HDMI ARC port, you should now be getting Dolby Digital / Dolby Digital Plus / Atmos sound. 

You may also need to go into Settings > Video and Audio on the Apple TV to check that everything is correct there as well. Select Audio Format, then under Immersive Audio check that Dolby Atmos is On (you’ll probably only see this option once you’ve made the changes in your TV settings). 

Good luck, and please report back how everything goes! 

I tried every TV setting I found, so far no luck.

Digital Sound default was Auto, and also tried Passthrough. The HDMI Input is set to Bitstream.

My guess is, that the Apple TV decodes DD and outputs multi-channel PCM

Apple TV outputs LPCM for uncompressed audio. I’m guessing that’s what it uses for DD+.  Sonos has announced that the Arc will support LPCM in the future via software update. The Atmos is delivered via MAT container I believe which the Arc can decode. 
 

You should get DD and DD+ via the tv internal apps though. 

I think it’s a little more complicated than that (as if this needed to be more complicated, right?). 

To the best of my understanding, if the Apple TV 4K is outputting uncompressed LPCM audio, then it’s already decoded the Dolby Digital / Dolby Digital Plus bitstream from the source. The TV would then think it’s receiving regular old PCM audio and then try to pass that along to the Sonos Arc. The Arc, not seeing a MAT container or a Dolby Digital Plus bitstream and not currently being compatible with LPCM should do absolutely nothing (unless the Arc’s current behavior is to take a multichannel PCM stream and simply ignore all but the Left/Right stereo channels which could explain the behavior @michary is seeing from his current setup). 

Since Dolby says “there is NO way of sending Dolby Atmos over PCM over HDMI, so you have to send Dolby Digital Plus or Dolby TrueHD to get the Dolby Atmos experience"(https://developer.dolby.com/blog/dolby-audio-over-hdmi-part-2-signaling-and-carriage/), then @michary will HAVE to get the Apple TV 4K to output either Dolby Digital Plus or, if people are correct in how the Apple TV 4K delivers Dolby Atmos, MAT.

I don’t have my Sonos Arc yet so I don’t have personal experience with this, but I still think my original advice is sound: make sure the television is set to correctly pass through audio on HDMI-ARC first, then make sure the Apple TV 4K is set to output Atmos. My assumption on that is that it’s basically working backwards through the Atmos chain: the Sonos Arc is connected to the TV and both recognize each other as Atmos-compatible devices (via some probably unhelpfully named and obscure setting buried in the TV’s menus). Then, once that connection is made, the Apple TV 4K can “see” that there’s an Atmos “receiver” at the end of the chain and gives the user the ability to select Atmos as the sound output (much in the same way it only presents the options for HDR or Dolby Vision if it detects that it’s connected to a compatible display).   

(Sonos, if you’re reading all this, this is why it would have been REALLY helpful to simply put another damn HDMI pass-through connector on the stinking Arc….)

@michary: Additional thought - Since every device in your chain is fully Atmos-compatible and uses eARC, maybe one or more of your HDMI cables is too old? During my research on this frustrating, but admittedly fascinating, topic, I learned that HDMI 2.1 / eARC basically adds Ethernet into the mix in order to provide the sort of massive bandwidth needed for all of this stuff. Maybe swap out both the HDMI cables between the Apple TV 4K and the TV and the TV and the Sonos Arc with certified HDMI 2.1 cables and try again? I know from experience that Apple TV 4K is VERY picky about the type of HDMI cable it is happy with for Dolby Vision output when I had to buy a particular Belkin cable  to prevent dropouts when I first got my current TV. I ASSUME that Sonos is including an HDMI 2.1 cable in the box with the Arc since eARC is such a bit part of the package, but what do I know? 

So first of all, I get Dolby Atmos on the Arc… but everything else from the Apple TV is Stereo.

Also a side info: When the TV is on passthrough in Sonos is shown “Dolby Atmos”. In “auto” it is “Dolby Atmos (DD+)”.. Which could indicate that the TV changes the audio-format.

To the best of my understanding, if the Apple TV 4K is outputting uncompressed LPCM audio, then it’s already decoded the Dolby Digital / Dolby Digital Plus bitstream from the source. The TV would then think it’s receiving regular old PCM audio and then try to pass that along to the Sonos Arc. The Arc, not seeing a MAT container or a Dolby Digital Plus bitstream and not currently being compatible with LPCM should do absolutely nothing (unless the Arc’s current behavior is to take a multichannel PCM stream and simply ignore all but the Left/Right stereo channels which could explain the behavior @michary is seeing from his current setup). 

I think with the HDMI-Arc the TV and Soundbar exchange which audio formats are supported. I would guess that the TV itself strips it to stereo.

Additional thought - Since every device in your chain is fully Atmos-compatible and uses eARC, maybe one or more of your HDMI cables is too old? During my research on this frustrating, but admittedly fascinating, topic, I learned that HDMI 2.1 / eARC basically adds Ethernet into the mix in order to provide the sort of massive bandwidth needed for all of this stuff. 

I used the bundled HDMI cable from the Arc.. so this one should be save..

The Apple TV 4K only supports HDMI 2.0a.. seems like a long-shot but I can try it 

Sorry, I’m not following. What are you getting Atmos on the Arc from? The Apple TV? The TV’s internal apps? And what do you mean by “everything else” from the Apple TV?  

Oh sorry, I should have make it a little clearer..

I only use the Apple TV 4K:

• Track in Dolby Atmos → I get Atmos on the Arc
• Track in Dolby Digital 5.1 → I get PCM (2.0) on the Arc

Ah, now I understand. Thank you! 

Unfortunately, though I understand what you mean, I still don’t understand what’s happening. ;-) 

I get your comment now about the TV “stripping it to stereo” which I would agree with. If the Apple TV 4K is outputting Dolby Atmos, then obviously everything should be good. I ASSUME (there’s that word again) that if the ATV is playing a Dolby Digital 5.1 source then it SHOULD be sending the bitstream through the TV and on to the Sonos Arc…. but Apple TV’s use of Dolby MAT (Metadata-enhanced Audio Transmission) may complicate that. 

Honestly, I’m not sure and have no way to currently test, so I may just be doing more harm than good here speculating. Here are a couple of interesting things I found trying to find an answer to this that may be helpful. If anyone can add more insight, please feel free: 

https://discussions.apple.com/thread/250698778

https://www.dolby.com/us/en/technologies/dolby-atmos/dolby-atmos-for-the-home-theater.pdf

I’m getting my C9 and my Arc delivered tomorrow so I’m guessing I’ll have all that fun over the weekend. Do you get DD when you’re using the internal apps ?

I don’t exactly know how Dolby MAT works, but can’t the Apple TV always send LPCM, and add MAT for an ATMOS signal ? 

And signal received by the ARC could be only PCM Stereo, plus MAT infos, resulting in “Dolby Atmos” indicated by the ARC ? 

This would explain the PCM Stereo for a DD signal, and Atmos (but just stereo Atmos) for an Atmos signal.