What service/program are you using for your source?

Your question is on spot, apparently non of my sources were Dolby Atmos. I downloaded a bunch of video I found on Google (demos) and some (not all) indeed were shown as Dolby Atmos on the app.

Given the fact I don't have a lot of Dolby Atmos options , which is better , dolby multichannel pcm 2.0 or

dolby digital plus 2.0?

Either one is 2.0 channels, so whatever sounds better to your ears. 
 

If you can get a source that can send 5.1, that would ultimately be better for your Arc.

I see. Thanks. Does tidal (music streaming) sends 5.1 / Dolby Atmos? 

No idea, I don’t use Tidal. Have you checked the Sonos FAQ on Tidal?

Nope. Will do

 Thanks

If you have or can borrow an Amazon Firestick it has a very nice Atmos test down in the setup menus. You can play the demo directly from:

Settings -> Display & Audio -> Audio and Video Diagnostics -> Play Dolby Atmos and Vision content

I kind of like it as a screensaver when things are noisy, the water noises really help mask the annoying noises.