Unplug the TV and Apple TV from power for a couple of minutes.

Good morning.in fact your TV does not seem to be compatible with 5.1 via HDMI.you therefore need to connect your optical soundbar to your Apple TV.Sincerely

You can NOT connect anything to an Apple TV other than HDMI that goes to your TV..
He has connected it correctly via optical cable to his TV. The same way I have connected my Playbar-Sub-2xPlay 1. (TV Samsung S90C)
(All mine run 100% on Wifi, no cables other than the optical cable and power)

Initially, I would also turn everything off, meaning remove all power to your devices. Wait approx. 5-10min and then switch on the Arc and other SONOS devices and finally your TV.. Make sure all settings are correct and then test it again. (your settings on Apple TV are correct)

You can also try changing settings on your Apple TV, remove 5.1 and then try changing it back to 5.1.

Thanks for suggestions all.

Tried powering off and unplugging all devices, waiting 10 min, then plugging back in and turning on (starting with Sonos). No joy.

Also tried toggling all the settings I listed in OP, no joy.

Frustrating because I know it’s possible. Was getting 5.1 with same system/setup just a month ago.

Your TV is pretty old. Are you 100% certain you were able to get 5.1 audio from the Apple TV? Do you remember what specific 5.1 source you were watching when it worked?

Yes 100% certain. Would typically see “Dolby Digital 5.1” on the Sonos App while watching. Apple TV 4K via Disney+ and Max. HDMI from Apple TV 4K to the TV, then optical audio port to HDMI/optical adapter (the one that came with the Arc), to HDMI cable that came with the Arc, to the Arc. 

I recall that last year when I was trying to get it to work, I had a similar problem only getting Stereo PCM, but then the right setting in the Apple TV enabled Dolby Digital 5.1. Don’t remember what that was. I feel like it was one of those settings to change Audio format output. 

When I lost the 5.1 about a month ago, it was coincident with me disconnecting the ethernet cable from the Sonos Connect:Amp (which is driving the surround speakers), and replacing that with another. I didn’t notice the change then, so I can’t say that was the cause, but that was the only thing I changed. Since around then everything is Stereo PCM.

Hi @davidagalvan 

Thanks for your post!

It’s certainly not unusual for older TVs not to pass surround sound from external devices to HDMI-ARC, so I would be inclined to take the word of your TV’s manual, but I suppose it’s conceivable at least that the manufacturer added support for passing non-stereo feeds to the connected HDMI-ARC device with a software update, after the manual was written. I highly recommend you contact Samsung to find out definitively.

Note that it was not unknown for these TVs to still pass surround sound to ARC when an internal source was in use - are you sure you didn’t only see Dolby Digital 5.1 reported in the Sonos app when using apps built into the TV? For me, this is far more likely than a TV whose manual says it will not pass surround sound to be doing so.

I hope this helps.

Thanks for your response. 
 

This TV has no HDMI-ARC port. Only optical audio out. 
 

yes 100% positive was getting Dolby Digital 5.1 to the Sonos Arc via the optical audio out port for the past year. Yes sure it was driven by Apple TV 4K. (This TV doesn’t even have any apps built in as far as I know.)

Maybe the new update will solve your challenges..

New software update 24/1-24

Hi @davidagalvan 

Understood, though this detail does not change my recommendation.

In your initial post, the quote from the manual mentioned DTV playing in surround - that was what I meant by an “internal source”.

I can only recommend contacting Samsung, still, for clarification.

I hope this helps.