Whether or not you can get Dolby Atmos audio to play through the Arc is more dependent on the source, media device, and TV than the HDMI cable. FYI… the Sonos-supplied HDMI cable that came with the Arc supports Dolby Atmos audio.

I would be surprised if the cable supplied with the Arc was incapable of carrying Atmos. I used a different cable because I had one that I knew was compatible.

Did you see any improvement?

There is no scope for improvement.  It either works or it doesn’t.

I’ve tried watching a film on Netflix confirmed as Dolby Atmos; Arc is connected to the Arc port on my Sony AF9. ( eArc-enabled tv) but I’m not seeing ‘ Dolby Atmos’ confirmation on the S2 app controller?

confused and frustrated!

The HDMI cable supplied with the Arc is definitely capable of transmitting Dolby Atmos audio. There will be no perceivable difference in audio by changing out the cable.

To get Dolby Atmos sound from Sonos Arc, you need to address the following:

1. Connections. Sonos Arc needs to be connected to your TV with an HDMI cord (one is provided with Arc) via an ARC or eARC port (eARC is required for Dolby TrueHD and Dolby MAT).

2. Compatibility. Every device that ‘touches’ the audio (e.g. Blu-ray player, Apple TV, Fire TV) needs to be able to pass Atmos on to the next in line—from the source of the content to Sonos Arc. For Blu-ray, the individual discs need to have Atmos content and the player needs to be capable of sending Atmos as well.

3. Apps. Your app (e.g. Netflix, Disney+) needs to be capable of playing Atmos content via your particular TV. 

4. Content. Your content needs to have Atmos audio in an Arc-supported codec: Dolby Digital Plus, Dolby TrueHD, or Dolby MAT. 

Take a look at this guide we wrote addressing Atmos and the Arc.

Very unlikely to be the cable.  Are you using the Netflix app on the TV or from an external source?  What Netflix subscription level do you have?  Have you managed to get it to play Atmos on any other source?  What audio format is your TV set to output?

Do you have a Premium subscription on Netflix?
 

Be sure your TV’s audio settings are set to these settings:

Audio Output: Audio System
Digital Audio Out: Auto 1
Dolby Digital Plus Output: Dolby Digital Plus
eARC Mode: Auto

My Netflix subscription is an ‘add-on’ through my Sky subscription. How would I confirm it’s a Premium subscription subscription, unless I speak to Sky please?

Netflix via Sky remote. I don’t have anything connected apart from Sky Q 2TB box and the Sony AF9

From this website: https://www.pocket-lint.com/tv/news/sky/145746-how-netflix-works-on-sky-q-your-questions-answered

If you have a Sky Q Entertainment subscription and the 1TB box you will get Netflix Standard membership (two streams in HD). If you have a Sky Q Experience (formerly known as Q Multiscreen) subscription with the 2TB box, you get Netflix Premium membership (four streams in up to Ultra HD).

Do you see the Dolby Atmos logo on the description of any of the shows or movies on Netflix? Did you set your TV audio settings to what I posted above?

Thank you all for your suggestions and support: ‘pass-through’ on the tv was not on ! 
 

many thanks once again

My app now shows Dolby Digital 

The Sonos app showing “Dolby Digital Plus 5.1” is progress. Now try testing the show Ratched on Netflix and see if it shows “Dolby Atmos (DD+)” in the Sonos app.

Not sure what I’ve done but it worked!

Thank you all once again

Glad you got it working. If you want to hear Dolby Atmos audio in all its glory, turn off Night Sound (moon icon) and Speech Enhancement in the Sonos app. Enjoy!