The Sonos Arc can only play the audio when it receives it - I suspect something is holding it up en-route from the Apple TV. The first thing I would perhaps try, is powering off all devices (not standby) and keep everything off for a couple of minutes to ‘drain’ the circuitry, check cable connections and power all back on again, starting with the TV, let things settle and see if that solves the issue.

As a side note, I have an HD Fury Arcana on the way, which I am confident will fix the issue by bypassing the TV for audio, but it shouldn’t be needed as eARC was supposed to make all of this work…

Yes, I agree, but that relates to the connection between the Arc and your TV/built-in Apps, which I think you mentioned do work okay. - The issue seems to point towards something between the Apps on the ATV 4K and the pass-through of the audio via the TV regular HDMI ports to its eARC port.

Maybe the TV is set to ‘auto’ for its digital-sound-out and is trying to ‘process’ the data en-route to the Arc. If the TV port (settings) have an option to pass-through the audio instead of ‘Auto’, I would select that option instead, if available, as such a ‘by-pass’ of the audio may greatly improve the delivery of the audio stream to the Arc.

The Arcana passes-through the audio stream received in the same way by by-passing the TV ports.

Based on the dozens of threads on here and other forums where people have very similar issues with Samsung TV’s in this setup, I believe the problem lies somewhere in the TV.  I have tried every combination of settings on the TV, and yes it is set to pass-through.  The Arcana will take the ATV audio and send it directly to the ARC by bypassing the TV.  That has been the most consistent “resolution” in all of the forum posts I have read.  I keep hoping for somebody with the same issue to post a fix or for Samsung to post a firmware update that addresses this.  So far, nothing…

Maybe speak to Samsung’s support desk, as there are often things they can do, or recommend you try - they may even have a firmware-update in the pipeline that may go onto fix things for you.

If the audio works in sync when using PCM stereo output, then that may show the TV is not passing through the multichannel LPCM (uncompressed) stream quickly enough from the Apple TV and see what Samsung have to say about that - Meanwhile, you could of course also try switching the Apple TV audio output to DD+ Atmos audio (compressed) too, just to see if that may improve things. 

I have same issues. Latest Apple TV4KHDR to new 65” QN90A Samsung NeoQLED to new Sonos ARC. Too frequently I lose audio when changing programs. But also has delays when using connected cable dvr box. We have best money can buy but poor technology. ???

I used HD Fury… too cumbersome and not reliable either. 

I have the same issue.  Setup is AppleTV Gen 1 > QN90A > Sonos ARC.

I have noticed by watching the Sonos App that my issue happens when the audio source changes from Digital to PCM to Digital and sound will drop on it’s last switch (back to digital).  I have an appointment with Samsung next Monday for a service tech to come out to my house, I will let you know what I find out.

So far I have tried resetting everything, tried Gen1 and Gen 2 Apple TV 4k, replaced cables, factory reset apple tv, and countless hours of support with all 3 companies have not produced results.  Samsung best solution was “stop using AppleTV because all the apps are available on the TV.  I really wouldn’t waste much time with tech support, nobody has been able to resolve the issue.