Sonos Arc no Atmos

What Dolby Atmos content are you testing? What streaming apps are you using? Is your Apple TV an Apple TV 4K?

Apple TV only does Dolby Digital 5.1. You may be able to get Dolby Digital Plus on an Apple TV 4K from non Apple apps, depending on the app, but the Apple apps will only do MAT. MAT can only be carried to your TV via eARC, and not standard ARC.

Tried Disney+, Netflix and native Apple TV movies. The Apple TV in question is a Apple TV 4K. 

Unfortunately, I don’t think your TV will pass through Dolby Atmos audio from your Apple TV 4K because your TV is only equipped with HDMI ARC not eARC.

I also don’t think Disney+ supports Dolby Atmos on Android TV.

The only option that could work is the Netflix app on your TV. But make sure you have a Netflix Premium subscription.

I do have Netflix Premium. I thought it would pass compressed atmos (re: appletv). 

I think Apple TV carries Dolby Atmos audio via Dolby MAT as opposed to Dolby Digital Plus. Dolby MAT requires an HDMI eARC connection. Have you tried Netflix through your internal TV apps?

I tried out Polar via Netflix on Android TV (native to my Sony 850E) and still only saw Dolby Digital Plus. 

Looks like your TV won’t allow you to get Dolby Atmos audio. I had the same problem with my seven-year-old Panasonic plasma. I purchased the HD Fury Arcana and now I can get Dolby Atmos from Netflix, Disney+, and Blu-ray discs. It’s $199.99 but worth every penny: https://hdfury.com/product/4k-arcana-18gbps/

On a side note tried Prime video via Android TV - Jack Ryan - Season two and it showed atmos. 

Well at least you now know your TV is capable of passing through Atmos audio from supported native apps. Unfortunately Prime has very limited Atmos content. Hopefully Disney+ and Netflix will add support for Atmos on your TV in the near future. Another app you could try is VUDU.