Be sure the TV is set to these settings:

HDMI-eARC Mode: Auto

Digital Output Audio Format: Pass-Through

And the Apple TV is set to this audio setting:

Change Format: OFF

Thank you! The last thing I tried was the passing-through setting. But with this setting the sound from Netflix, HBO Viaplay, disappeared totally. 
Any idea what could be the reason?

Samsung TVs with eARC are known to have issues. Keep the setting set to Pass Through and then do this:

Disconnect everything from the TV and unplug the Beam and TV from power for a couple of minutes. Plug everything back in but keep everything disconnected. Make sure Anynet+ is enabled in the TV settings. Run TV Setup in the Sonos app and follow the app instructions. When the app instructs you, connect the Beam to the TV’s HDMI ARC/eARC port using the Sonos-supplied HDMI cable. After the Beam has been successfully connected, connect the Apple TV  to the TV and test again.

It did not help, unfortenately. But thanks!

I did as you said, except for the HDMI cabel. I could not use the Sonos cable as I had to use a longer one. But it’s a good one.

But I noticed that everything works great with Samsungs own apps, Netflix , Viaplay etc with passing through. Both Atmos and 5.1.  Appletvs apps works with Atmos and the 5.1 films only deliver pcm 2.0.

Strange it is!

That is strange. The only other thing I would try is rebooting the Apple TV and making sure the Apple TV and Samsung TV have the latest firmware/software installed.