YouTube is mostly stereo, so that could be why you do not get any sound from the rear speakers. What sound format does your app say you are playing? 
 

Its Youtube Live.  It says Dolby Digital 5.1

The TV when I turn on says Dolby Atmos on top right and when I click on Youtube TV live app from the Apple TV 

What does the Sonos App show is being sent to the Arc?

I have found many times that the source says 5.1 or Atmos but what is sent is actually just stereo.

It could be a TV setting too, but we need to know what Sonos thinks it is getting sent.

Dolby digital 5.1

Well I hope Apple TV comes out with a better device cause I cannot afford Sonos ERA 300s any time soon!

Do you get Dolby Atmos on the other Apple TV streaming apps? You’ve checked the sound setting on the Apple TV.

It could just be the content you’re watching?

Yes… I just think the ERA 100 aren’t as good as others

There's some 5.1 surround sound test videos on YouTube. Even though Sonos reports stereo they use some sort of Dolby pro logic to hit your individual speakers. You should hear the 100s easily. They hit my Play:1s no problem along with my fronts.

I have YouTube live it’s like cable tv

Maybe you are expecting too much sound from the surrounds? Surround sound is helicopters flying away, applause with concerts, birds in a park, that kind of thing, you could compare YouTube live to Netflix though.

Hi ​@pag262007 

Welcome to the Sonos Community!

Sorry to hear of this issue you are having with playing YouTube via your Apple TV.

If the issue you experience does not seem related to the content you are watching, and if you see the same behaviour with other sources, I recommend you get in touch with our technical support team who have tools at their disposal that will allow them to give you advice specific to your Sonos system and what it reports.

I hope this helps.