Sonos Arc Ultra not playing Atmos

My TV model is a year older than yours (A80J), but the menu and settings are probably the same. I also have a universal remote to control everything, so CEC (Bravia Sync) is turned off. I’m using a Beam 2 on this TV and Dolby Atmos works. Here are my settings:

Display & Sound → Audio Output
Sound Mode Sync = On
A/V Sync = Auto
eARC Mode = Auto
Digital Audio Out = Auto 1
Dolby Digital Plus Output = Dolby Digital Plus
Pass Through Mode = Auto
 

While watching something with Dolby Atmos, look at your TV now playing screen in the Sonos app to see what audio format is playing. Also look in Gear Icon → General Settings → About Your System → scroll to your soundbar: Audio In  

If the Sonos app is showing Dolby Digital Plus 5.1 instead of Dolby Atmos, then the Arc Ultra is not receiving an Atmos signal. Since the soundbar is connected to the TV's eARC port and your Sony A80K supports Atmos, the issue is most likely related to the source content, streaming app settings, subscription tier, or TV audio pass-through configuration. Make sure the content actually includes a Dolby Atmos audio track, that Atmos is enabled in the streaming app and source device settings, and that the TV's eARC and pass-through options are set correctly. The Sonos app generally reports the audio format accurately, so if it shows Dolby Digital Plus 5.1, that's likely the format currently being sent to the soundbar rather than a firmware or app bug.

Having a "known good" Atmos source is handy, once you know what is being sent to the TV you can troubleshoot where downstream from your source things go wrong.

In my case I use an Amazon Firestick that includes an Atmos test source. Not all Firesticks have it, the 4k Plus and 4k Max do, not free but for $25 or $35 worth it as a troubleshooting tool.

Cinematic experience - Watch in vibrant 4K Ultra HD with support for Dolby Vision, HDR10+, and immersive Dolby Atmos audio.

https://www.amazon.com/dp/B0F7Z4QZTT?ref=amzdv_ucc_dp_lod_B0C6W3D4RM_B0F7Z4QZTT

Other plug-in streaming devices might have the same test function.

Thank you for your inputs. Cross checked against mine. 

Display & Sound → Audio Output

Speakers = Audio System

Audio System Prioritisation - On (this was Off)

Sound Mode Sync = On

A/V Sync = Auto

eARC Mode = Auto

Digital Audio Out = Auto 1

Dolby Digital Plus Output = Dolby Digital Plus (this was Dolby Digital). Not sure how & when this one changed, but have got it back to Dolby Digital Plus

Pass Through Mode = Auto

With the 2 changes that I did based on your inputs, the content I played, in this case Grand Turismo from Sony Pictures Core & Netflix, my soundbar shows:

1. Dolby Atmos when playing content from Sony Pictures Core

2. Dolby Digital Plus 5.1 when playing it from Netflix

Netflix is set for highest quality. 

I guess this answers my original question and probably points to the way the streaming app is forwarding the audio content.

I believe we can close this thread.

Thanks once again kdowling and everyone for stepping in & helping guide me through this. 

Yes it is related to the source, the streaming app. 

Yes it is related to the source, the streaming app. 

Thanks you for pointing this out & it's something that I am now considering to have as a backup.