Display correct sound quality

Sonos only shows what it is being sent, have you confirmed the Amazon changes to the Atmos availability option? Hidden under Commercial Free if I remember correctly.

If you have done that confirm you are on the latest (awful) Firestick firmware followed by a power cycle of the Sonos, Firestick and TV. 

Thank you for your answer !

I didn’t touch anything since the last time I played atmos content. The firestick has the latest update. 
 

But Amazon could have changed something, and likely did. They have been messing with their plans (eg 4k only for ad-free).

Actually, Monarch is on Apple TV, so it shouldn’t be affected by the changes to Prime Video.  Though I have noticed if you are purchasing a service through Prime (aka “Prime Channels”), the content available through the Prime app (or the main menu of a Fire TV device) sometimes lacks the features of the same content available in the app for the service itself. 

For instance, in the Paramount+ app, Top Gun: Maverick is available in HDR 10 and Atmos.  Through the Paramount+ Prime Channel, there is no HDR and no Atmos for Top Gun: Maverick, just UHD.  I don’t know if this discrepancy carries over to the Apple TV app vs. Apple TV via Prime Channels, but that could be what the OP is seeing. If I were the OP, I would load the Apple TV app to my Fire TV stick and try viewing Monarch through that to see if it gives Atmos. 

 Tried Monarch… on the FireStick using Apple.  It shows as Dolby Atmos.  Tried to watch it on Amazon Prime which stopped me in my tracks because of some stupid PIN number I don’t ever remember setting up.  Anyway… the description showed UHD, not Dolby Atmos.

 When I look at the description of Monarch… in Apple it shows Dolby Atmos.

 Now I gotta work around that PIN number thing.

Thank you all for your answer. 
 

That’s crazy but Ive tried another time now and the app Sonos display correctly Dolby Atmos like before. 
 

I have no explanations…

PS: I show Monarch via Mycanal app in Amazon Stick 4k

cheers