If you are only using the TV’s native streaming apps, set the HDMI-eARC Mode setting on the TV to OFF. Keep Digital Output Audio Format set to Pass-Through.

Thank you for your reply. I mainly use the Sky Q box for TV viewing. With pass through selected this is when the distortion occurs.

Is PCM a lesser quality audio? If not then I'll stick with the Auto setting on the TV 

With the HDMI-eARC Mode set to OFF and Digital Output Audio Format set to Pass-Through, do you still experience the distortion?

Watching a typical show, what specific audio format is displayed in the Sonos app with each setting (Auto/Pass Through)?

I've just changed the settings as you described. Watching a channel 5 prog the Sonos app shows Dolby Digital 5.1. I did also test an Atmos film on Prime and that showed Dolby Atmos on the Sonos app.

With the TV set to Auto it would show Dolby Multichannel PCM 2.0 or 5.1

I stick with your suggested settings for now and see if the distortion comes back, hopefully not !

So after a number of days the distortion is still there sadly. For the first 12 hours or so it was fine but now as it was. If I pause the TV briefly and then play again the distortion goes but only for a short time. I've reverted back to Auto on the TV and not pass through. Like this zero distortion. All very strange. 

If you search online, you might perhaps see that earlier this year Samsung had some issues with some of their TV models and attached soundbars (different brands, aswell as Sonos). In September they issued some firmware updates for various TV models, but some Samsung users continued to highlight issues. Sonos then also released the 15.8 update and that was alleged to be causing some issues with some models of Samsung TV’s too. It was thought the later 15.10 Sonos update fixed the issues, but some users are still reporting problems particularly with eARC enabled on the TV, but it mostly relates to the TV audio dropping out completely, rather than the sound just being distorted.

Anyhow, Sonos Staff have said in other threads here in the community that if you’re still having issues, to first check that you’re using the latest TV and Sonos updates and have swapped-out cables etc; to then reproduce the problem and submit a Sonos system diagnostic report from within the Sonos App, note it’s reference and contact/chat the Sonos Support Staff via this LINK and provide them with the detail and see if they can perhaps assist to rectify the matter. So it might be worthwhile reporting your issue and see if the Staff can perhaps assist.

An update and hopefully the final update. I swapped out the Sonos supplied HDMI cable which made no difference. What has sorted the issue is changing the eArc setting on the TV from Auto to Off. The Audio output is set to Auto and now I always get Dolby Digital as shown on the Sonos app. This of course varies depending on the audio on the channel or program we watch. When available it does give me Atmos. 

It's been a few days since I made the changes, so far, so good. 

It would be interesting to understand why the settings adjustment has cured the problem. 

Thank you to those that commented 

 @Parsen66,

For quite some time a few Samsung TV models have ‘allegedly’ had some issues with the eARC setting enabled and if you look back through the various community threads here and elsewhere online, you may perhaps see that other Samsung (and soundbar) users have suggested leaving it disabled with various makes of soundbar and to just use the connecting port as an HDMI-ARC port or even to use the TV’s optical port in some cases, albeit to a much lesser extent. It’s not clear if it’s a hardware, or software, issue or a ‘problem’ with the soundbar software etc.

Anyhow, here is a quick comparison (attached) between ARC and eARC capabilities, not sure it really helps though, as my Sonos soundbars are working with Sharp & 2 x LG TV ports set to eARC, so its an issue mainly with  some Samsung branded TV’s, but it’s not (yet) clear where the ‘real’ issues may lie ?