On the app (on the now playing screen), what wording does the room speaker audio show when playing a movie that you know to be in 5.1 surround? Does it say “5.1 Dolby” or something else like “PCM Stereo”?

What level have you set the surrounds to in the app when listening to the TV? (Settings / Room / Surround Audio / TV Level.) Have you got the levels too low?

Something to remember is that your surround speakers will be silent unless there is a sound in that scene that specifically needs to utilise that particular surround speaker. You won't hear sound there all the time, like you do from the Playbar. You’ll find they are mostly just very faint atmospheric noise.

But when playing music, you will hear sound from the surrounds which mirrors the Playbar because it is simply stereo - so left and right on the Playbar will be the same as left and right surrounds. 

EDIT: Also check that the audio settings on both your TV and your Apple TV box are both correctly adjusted for 5.1 Dolby Digital (on the TV, you might need to have a certain setting for the optical audio out that allows it to pass through a 5.1 signal. But if the Sonos app is already showing “5.1 Dolby” then it’s all passing through fine). 

I have an issue with no sound from my rears (Ear300’s and Arc). discovered that they were no longer acting as surrounds. The new app had disconnected them and no amount of resetting or Sonos support has got them back. They will works as a stereo pair.

Thanxalot, Rhonny. 
You hit it right on the spot: the audio-pass-through on the TV had two positions. I switched it, and the 

TV-sound SPDIF PCM 2.0 changed to
TV-sound SPDIF Dolby Digital Surround 5.1

Voila! Everything is fine, now.

Can I have one more piece of advice?

I have two choices for my back speakers: either 60 cm above the floor or 200 cm above the floor. I can’t try it out before the decision because the top position requires that I install brackets in the window frames to carry the speakers, and they leave ugly screw holes if I subsequently remove them again. On the other hand, I bought those brackets precisely to place the speakers there. So what I'm really asking is: are there obvious reasons NOT to mount the speakers up high?

A secondary pair of questions: Can SONOS leverage this location positively for ATMOS if I should decide to upgrade my Netflix subscription to include ATMOS? And is the old SONOS Playbar actually able to handle the complex ATMOS signal?

Thank you again for your help!

Knud
(Denmark)

Hi! Glad I could help. 

The surround speakers should not be placed up high. They should be at, or just above, ear height when seated. 60cm is a little low perhaps, but 200cm is far too high. You won’t enjoy a surround effect with them up there as the sound will jar. Surround sound needs to be largely on the same horizontal plane as the Playbar, ideally. 

Placing the speakers at 200cm will not help in the future with Atmos either. Atmos works by firing sound up to the ceiling and bouncing it off to reach your ears (or having speakers in the ceiling), giving the impression of height. Putting your One SLs up there would - as for 5.1 - ruin the surround experience and do nothing to add Atmos.

You will not get Atmos from the Playbar. It is not Atmos-capable. You’ll need a Sonos Arc for that. And ultimately you’d need to upgrade the One SL surrounds to Sonos Era 300s. But certainly an Arc and 2 x One SLs and a Sub would do a very decent Atmos job (you just wouldn’t get any up-firing from the rear One SLs). 

Hope this helps.

EDIT: Your current TV won’t support Dolby Atmos either. The Apple TV box needs to connect to an HDMI port of a higher specification than your TV has. And a Sonos Arc needs to connect to an HDMI ARC or eARC port, which your TV also doesn’t have.