So, you’re saying it works fine on some ‘channels’, but not on others? Generally, that means the issue isn’t the Sonos, but the kind of signal being sent to it. For instance, the vast majority of content on YouTube has been modified when uploaded to be in stereo, which wouldn’t have anything playing in your Era 300s, just the front right and front left in your Arc Ultra. 

You’re saying it doesn’t work when you’re watching the Netflix app. Is this an app on your TV, and not your cable system? Have you checked to see what Sonos is receiving? The surrounds will only be working if the Arc Ultra is receiving either Dolby Digital, or Atmos. If the signal being received ( which could depend on the show, the app’s settings, or even the subscription type ) is only stereo, again, the surrounds won’t be playing any sound.

Essentially, the Sonos doesn’t have any knowledge about the source (which channel you’re watching), only the type of audio signal it is receiving. If it’s working in some areas, it is set up correctly, and where it isn’t working, you need to be looking at the source data. 

Thank you for taking the time to reply to my concerns. I have Sonos speakers in the playroom which is composed of 2 Sonos five speakers, an arc soundbar, and a subwoofer in the surround system. It plays youtube, Amazon, Netflix and Tv cable perfectly no issues at all. Everything is working perfectly. I recently bought the 2 era  300, an ultra arc soundbar and a subwoofer and set them iin souround system. For this set the tc cable channels sound plays to the sound bar and the era 30. When I switch to youtube from the TV app, Netfliz, Amazon etc  the era 300 turns silent. How to fix the sound issues content coming from youtube? My surround is set to Ambient. Or may be I should returned the era 300 and buy the Five?  Since I am not having issues with the five in a different room

Never use YouTube to test any kind of surround sound.  99.9% of the videos listed as Dolby Digital or Atmos are downgraded to stereo PCM when they are uploaded.  So you can’t trust them.  Bottom line, don’t expect YouTube to be “fixed”, it’s just how it is.

Next, the “Ambient/Full” setting applies to music being streamed via the internet.  It has no effect on sources coming in via HDMI, aka anything from the TV.

As to the other apps (Netflix, Amazon, etc.), as described above, you need to look at the Now Playing screen for the Arc/300/Sub room name.  The audio format will be listed on the screen.  If you are not seeing Dolby Digital or Atmos for those apps and you know the content is DD/Atmos, then your TV settings need to be adjusted. Consult your TV manufacturer for the proper settings to get DD/Atmos to a soundbar.  Note: A premium subscription from Netflix and Amazon iis required in order to stream Atmos content.

What is the model of your TV? There may be some sound settings that need to be adjusted in order to get the surrounds and sub to play.

Are you using the HDMI cable that came with the Ultra?

You cannot set the surround sound to Full when the TV is the source. It can only be set to Full when listening to music.

Thank you both.it makes sense now. My TV model is a Samsung Curve. It looks like the Sonos Five dlcan play Bothel stereo and Dolby digital, the Era 300 only the Dolby digital.