Perhaps try powering off both the TV and Arc for 30sec’s ...the two devices may need to do a ‘handshake’ to get the correct audio codec recognised, particularly now you have changed the settings on the TV between PCM to Dolby Atmos. I would have thought that the auto setting should be fine, but if not, then try the Dolby setting.
 

It’s quite important to do the initial connection between the devices when they are both powered off for the handshake to occur correctly, as mentioned in the Panasonic support video on their website, so hopefully that will fix it for you.

Thanks Ken I will try that when I get home from work. 

So I am correct in thinking that the tv should be able to decode/passthrough the Atmos signal from the native apps to the sounder then. I was starting to think that maybe the TV itself couldn't passthrough the right signal which didn't make any sense especially with the eArc compatibility between the two products and I may need a splitter/convertor of some kind to sort this out.

From what I read briefly on the Panasonic site, I am quite confident it will work with Atmos audio-out to the Arc no problem - the auto audio out setting should be fine you just need to follow their video in the support section on their website to switch off its internal speakers and enable HDMI-CEC - I assume that’s been done already, so powering the devices off for about 30 seconds should sort it, hopefully. Then try playing an atmos encoded Movie.

Just to also assist, you should be able to see the Arc’s currently playing audio by going to the Sonos App "Settings/System/About My System” and find the ‘Arc’ in the device-list and look at the entry next to Audio In.

Well I have tried what you asked Ken and it works for a short period of time. The audio is playing in 5:1 but then the audio starts to break up quite quickly and then it disappears again! To be sure it wasn’t a damaged HDMI lead or a problem with an Ethernet connection I have replaced both but it’s still the same. The weird thing is it only happens on the prime abs the BBC iPlayer native apps. I have apple 4K tv and everything on there output perfectly. I do prefer the picture quality on the native app hence trying to sort this out. 
 

i don’t suppose anybody else has encountered  a similar problem on their tv? 

How about if you switch off HDMI-CEC on your ‘other’ TV HDMI connected devices (including the AppleTV-4K) - or you perhaps unplug them temporarily, does that fix the issue with the in-built TV Apps?

Hi Ken, I only have one other input into the tv apart from the Sonos to the arc and that is the Apple TV. Yesterday I did a factory reset of the Sonos and unplugged the Apple TV to test if that was somehow hindering things but it didn't make a difference, still the native apps have either a really broken up sound  initially then shortly afterwards no sound at all. Weird.

Do you have another HDMI 2.1 cable to try?

What happens if you use optical out of the TV and the supplied adapter (just as an experiment, I know it isn’t a solution)

johnjo569,

I assume you have checked and ensured you have the latest TV firmware installed… it’s strange the Apple TV apps all seem to work okay, but the TV in-built Apps are having issues. I would maybe run the issue past the Panasonic Support desk to see what they may have to say about the issue. 
 

It would also be worthwhile establishing if switching the TV output to PCM stereo fixes the issue, or if the audio still drops out with that codec too.
 

It might be Panasonic are aware and looking into the matter, but if no joy there, then I would submit a Sonos diagnostic report when you next encounter the issue and post its reference back here, then contact/chat with the support staff via this LINK

Just for future reference, it is really unhelpful to have two parallel threads running on the same problem