What specific Samsung TV model do you have? What specific Dolby Atmos source are you testing on the Apple TV?

Samsung 65 Inch Q80A QLED 4K Smart TV (2021) - HD QLED TV With Wide Viewing Angle Boundless Screen, Alexa Built In, Smart TV Streaming, 100% Colour Volume Picture, Ultrawide Game View, QE65Q80AATXXU

Try this:

Disconnect everything from the TV and unplug the Arc and TV from power for a couple of minutes. Plug everything back in but keep everything disconnected. Make sure Anynet+ is enabled in the TV settings. Run TV Setup in the Sonos app and follow the app instructions. When the app instructs you, connect the Arc to the TV’s HDMI ARC/eARC port using the Sonos-supplied HDMI cable. After the Arc has been successfully connected, connect the Apple TV to the TV.

Then set the TV to these settings:

HDMI-eARC Mode: Auto

Digital Output Audio Format: Pass-Through

Set the Apple TV Audio Format settings to this:

Change Format: OFF

Dolby Atmos: ON

Did what you said. I have a Samsung tv and not Apple. I was only watching a show on Apple tv (the subscription service like Netflix). The sound is better I think but the Sonos app still says HDMI and PCM2.0 not Dolby! 
could it be something to do with the Tv? 

Stream the movie Greyhound with Tom Hanks on Apple TV+ and tell me what audio format the Sonos app displays.

What other streaming apps do you use?

I do get good audio and if I go closer to the era300s, can hear them as well. But the app still reads HDMI and PCM 2.0. 
I have Netflix, Disney+ and Now, other than Apple TV. 

Test the film Black Panther on Disney+ and Don’t Look Up on Netflix.

Still same for both movies! 

Try setting HDMI-eARC Mode to OFF on the TV.

So just noticed it was off already. Turned it back on. What should it be? In both cases, it says HDMI and PCM 2.0 

What is the Digital Output Audio Format setting on the TV currently set to?

Pass through and the delay as 130 

Very strange. Samsung TVs are the worst. One other thing to try is setting the Digital Output Audio Format setting to Auto instead of Pass Through. If this doesn’t work, you might consider getting a streaming device like a Roku Ultra or Apple TV 4K.

Might replace the TV in a few weeks. What company has the best integration with Sonos? Is it worth talking to Sonos customer service about this? 

LG and Sony seem to work well with Sonos. I have an LG C3 OLED that works perfectly with my Arc setup.

Does your TV have the firmware installed?

It wouldn’t hurt to try contacting Sonos Support. 

2020 Samsung TV works very well with Arc, sub and surrounds here…

Ensure you’re using a high-speed HDMI 2.0 cable.

Sound output: Receiver (HDMI-eARC)

Sound Expert settings:

    HDMI-eARC Mode - Auto

    Digital Output Audio Format - Pass-through*

    Dolby Atmos Compatibility - On

General - External Device Manager - Input Signal Plus - ensure all HDMI ports are set to ON.
 

*only useful if you have another box (eg Sky, Apple) connected to the TV as it passes the box’s sound through untouched. Otherwise if using the TV’s built-in apps, stick to Auto!

As well as my suggested settings (above), do you pay for the premium tiers on Disney, Netflix and Now? If you are on the basic tier for Disney and Netflix, and also don’t pay for Boost with Now, you might only get PCM 2.0 (eg stereo). Also most TV transmissions are only stereo so the majority of the time for ‘normal’ TV you will see PCM 2.0.

Not sure why there’s no 5.1 for Apple TV+ though…

But try my settings first!

Changed the settings as mentioned above and played Batman dark knight on Now and now it shows it as Dolby MAT 2.0. I was expecting something more along the lines of 5.x or 7.x. Any other settings to check? 

I get Dolby MAT 2.0 when watching footie on terrestrial ITV HD. So I think you’re set up correctly. It just sounds like you’re not getting 5.1 from your apps. Are they all up to date?

Do you have free, basic or premium tiers for your apps…?

I have the ultra HD package on Sky (box). I only get a PCM 2.0 when watching BBC or ITV. Checked - premium versions of at least Disney+ and Apple TV+. Netflix says it’s the ad free version so I am guessing it is the premium tier. Not sure what else I could try.