The implication seems to be that it’s happening when stereo is being sent, not a 5.1 signal. Which is unusual, and I’m sure frustrating. Have you rebooted your Arc and the Era speakers?

And the tv source, too?

Not sure…I was thinking the apps in use were through the Apple TV, but the OP says it’s not happening on Apple’s streaming media, which I took to mean videos using Apple’s players. No response yet, though. 

This is only with tv/movie content and definitely 5.1 not stereo. 

 To clarify. 

Hardware - Apple TV 4k

Apps being used that exhibit behavior- Youtube TV, Plex

Apps where behavior is not exhibited - AppleTV

If the Sonos is working with some apps, and not others, that points the finger squarely at those apps, and not Sonos directly. I’d be checking the settings for these apps to see what they’re ‘sending’ out through Apple TV’s HDMI to the TV set, which then creates the ARC/eARC signal to send to the Sonos. Technically, Sonos has no real idea what is sending the signal, per the CEC spec, it just publishes the fact that it’s an audio sink, and what formats it can process to the ‘computer’ in the TV that is the CEC ‘master’ device

That being said, I have low confidence in YouTube’s app actually sending a true 5.1 signal, but that is based on their web viewer, and all of the videos that claim to be in a 5.1 format, but are actually just stereo.

So how can I troubleshoot this? Everything was working perfectly until the past couple months. 

Just saying its the apps fault doesn’t ring true from anecdotal evidence. These issues happened at the same time with Plex and YoutubeTV where neither problem existed before. 

I can give you a rundown of the different media types and codecs etc. I have direct play via Plex so if its not decoding properly it could make sense that its a plex problem but why did this problem start happening in both apps, with multitudes of media.

Also I need to correct you. Its not youtube its Youtube TV app which does indeed have 5.1

Here is current behavior. I am currently watching TNT. NBA Tip-Off was on and the dialog sounds like its coming from left rear speaker. Now the actual game comes on and the dialog is playing from the center as it should be. 

This is the behavior I have been experiencing for a couple months now. Please help me understand how I can troubleshoot. Steps I can take, info to provide etc. 

Failing a reboot of the Sonos, (and I wouldn’t expect that to help), I would recommend that you submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it.

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.

When you speak directly to the phone folks, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.

But the proof that it isn’t a Sonos issue is in that fact that the Apple TV app works fine. As previously stated, checking the output in the settings of those apps seems most likely to resolve the issue. Oh, it may be worth rebooting the Apple TV, too.