Most YouTube videos are changed to stereo, for storage space considerations by YouTube. My understanding is that it’s only the TV subscription provides potential ‘larger’ files. Anything uploaded by a common ‘user’ is suspect. I don’t think they really know, I suspect it is explained in the user agreement, which of course, no one reads. 
 

Some things on regular YouTube such as 

Are in 5.1.  More things on YouTube TV are in 5.1, but it varies.  Things you would expect on Amazon, such as Movies, are in Dolby 5.1.

In my experience, even the videos marked as DD 5.1 or Atmos are downgraded to 2.0 PCM.  There are very few (if any) on the free subscription YouTube that are actually 5.1 or Atmos, even when marked as such.

Within YouTube, if I change from a typical video, I can see my Sonos iPhone app audio signal change on my screen from Dolby Digital 2.0 to Dolby Digital 5.1.  After giving up on support and playing with different combinations, Dolby Digital 5.1 is now working with my Play base, Sub (Version 2), and latest iteration AMP, running the rear surround channels.

Playbase

Connection

An Ethernet cable, wired to your router, is required to allow Wi-Fi to be disabled.

Disable Wi-Fi

Enable Wi-Fi    ✔️

Amp

Connection

An Ethernet cable, wired to your router, is required to allow Wi-Fi to be disabled. 

Disable Wi-Fi

Enable Wi-Fi    ✔️

Sub (Version 2)

Connection

An Ethernet cable, wired to your router, is required to allow Wi-Fi to be disabled. 

Disable Wi-Fi

Enable Wi-Fi    ✔️

​@Scott Gibbons When I watch the YouTube video you link I get this:

This person seems to have had the same issue that you are reporting, but was able to resolve it by changing settings within their television. See Sonos case below.

app shows stereo pcm / 2.0, but using eARC and surrounds

Hi

I have a sonos Arc and 2 Sonos one SL as surrounds 

The app says its playing stereo pcm and in the settings that the arc is receiving stereo 2.0

However, I'm running a brand new 55 inch LG oled TV and the arc is connected via a eARC hdmi cable.

The app shows that the audio is set to surround and the rear speakers are working.

But when I stream dolby atmos from Disney plus the app still says its receiving stereo

So ... I have connected it properly, I'm streaming dolby atmos, the TV is running the audio out correctly to the arc sound bar, the rear speakers are connected correctly, the app shows the sound mode is set to surround,  but on the about my system and the player on the app it says clearly its running stereo pcm…

How do I get this set up to run atmos?!?!

Pinned reply

Hi buddy,

Its a cx series LG OLED

However, after some exploring I found there were some additional setting in the tv's advanced sound settings that had not activated,  ... I changed them to dolby digital plus and pass through and it seems to have worked!

Full atmos and also 5.1 on other services 

Job done…

Tbh,  I wouldn't have worked it out if I hadn't been reading for round in these forums.

Thanks chaps

Moderator edit: removed picture showing personally identifiable information and increased legibility by removing platform metadata and adding link to thread.