Go to Settings → System → About My System. Scroll to ARC and look for Audio In:. This will show the audio format that ARC is receiving. Here you can check that the TV is sending a compatible surround format.

Describe your setup… What model TV do you own? Are you using any external media devices? What streaming apps are you using? What content are you testing?

First of all thanks for the replies.
I have a Panasonic JZ1500 as a TV.

• I tried the tv with the normal channels.
• I tried a 7.1 surround demo with the Youtube app
• I tried a movie with the Playstation (here I can set both PCM and Bitstream DOLBY or DTS).

In none of the cases have I ever heard surround sound with the effects behind, sideways or overhead that ATMOS should have (simulating of course).
Could it be a calibration problem without Trueplay?

All YouTube videos play in stereo, not in 5.1 or 7.1 audio. Don’t use YouTube videos to test.

What model PlayStation do you own? What movie are you testing?

Can you post photos of the current audio settings on your TV?

If you have just the Arc -- without surround/rear speakers -- you’re not going to hear anything from behind. You may be lucky enough to hear something from the side and overhead, depending on room reflections, but it could well be tenuous. 

I suspect you’re expecting acoustic miracles from the Arc.

Thank you guys! Ok, so fixed why I can't hear a 5.1 surround demo or higher (because Youtube is 2.1 only). Out an option.

I own the Playstation 4 (I saw that there are options to set "LINEAR PCM" - "BITSTREAM DOLBY" - "BITSTREAM DTS".

Yes, I only have the SONOS ARC without a speaker. I understand that I can not pretend as if I had a complete system with real speakers, but advertisements and various variations spoke of "enveloping audio" with effects that are demonstrated even if virtually, thanks to the fact that it echoes in the walls of the room) above my head. I am aware that it is not the same as having a real system (unfortunately I do not have the space), but not to hear any spatial surround effect I do not think it is good …

Last thing (here is the reason for my "alarmism"): I just dropped a BOSE 700 because I consider SONOS superior. So I have an experience until a few days ago with BOSE, and I assure you that having the same devices I have now (PANASONIC JZ1500 TV and PLAYSTATION 4) I could hear the various effects very well.
Stupid example: if there was a police car coming from left to right, I could hear very well in the left ear that it was coming from that very left side
If there were birds chirping from the right side, I could hear it coming ONLY FROM THE RIGHT SIDE.
On the other hand, with the new SONOS ARC the sound is always static, it doesn't move! So it only serves to amplify the audio!
Again, with the BOSE (which I consider inferior) I had these enveloping effects with the same devices with which I am using the SONOS from today.
Could it be because of the Trueplay that would have configured everything or not?
Unfortunately, not having iOS I have no way to use it ...

On the PS4, if you are watching a Dolby TrueHD or Dolby Atmos disc, set Audio Format (Priority) to Bitstream (Dolby). If you are watching a DTS-HD Master Audio or DTS:X disc, set it to Bitstream (DTS) or Linear PCM.

Also, while the Blu-ray is playing, press the Options button on the PS4 controller. Select Settings and set Audio Format to Bitstream (Direct).

To confirm that you are getting 5.1 audio, the Now Playing screen on the Sonos app will display the audio that is currently playing. It should display “Dolby TrueHD 5.1”, “Dolby Atmos”, “DTS Surround 5.1”, or “Multichannel PCM 5.1”.

Try to borrow an friend or family member’s iOS device and perform Trueplay tuning. It only needs to be done once.

Yes, all done! A thousand thanks!
Selecting "Bitstream (direct)" from the Playstation increases the volume of the overall sound, which is already an advantage! ;-)
The problem of surround effects remains the problem. I don't feel the cinema effect and the spatiality of the sound ...
With the same devices (TV and Playstation) the BOSE soundbar had all these surround and spatial effects and were always heard (for example I could hear the side sounds in the ear very well when there was a scene of a helicopter coming from one of the two sides ...). Now with the SONOS ARC I hear the helicopter only centrally, without that spatiality I am talking about ...
So it's not a problem with the TV or Playstation settings.
The only hope (but tell me if this could be the probable problem) is the speech that I have not yet calibrated the sound with Trueplay.

While watching the disc, what specific audio is displayed on the Now Playing screen on the Sonos app (see example below)? What movie are you watching? Is it on Blu-ray?

Yes yes, the film is in bluray and (as you suggested in the other answers) I checked and gives both 5.1 and Dolby Atmos 7.1 correctly.
But the "spatiality" effect is not there ... But then again, I haven't done the calibration because at the moment I don't have an iOS where to download the app ...

FYI… to hear the height channel effects the best from Dolby Atmos content, ideally you want to be in a room with 8-9 foot flat ceilings and sit about 5-7 feet away from the Arc. Also be sure to adjust the Height Audio level higher under the Surround Audio settings in the Sonos app. I have mine set to +6.

Have a nice Sunday! Thanks for everything!
I'll try later (a friend finally comes home, so I do this via iOS and let you know and update you).
A curiosity please: since you have set everything well (I imagine even after having studied everything well, given your great preparation ...), what can I expect as surround effects, even though I am aware that we always talk about "virtual audio" why don't i have physical speakers? Can I expect top-down audio, side-to-ear audio? For audio from behind your head, I guess I need the physical speakers placed behind it ... Although the demo from the SONOS website demonstrates the ability to wrap 360 ° audio with the soundbar alone … 

Much of the experience is under the control of the video’s director and the expectations of the viewer. New surround users expect to be smacked in the back of the head by the surround and will be disappointed if the first film they play is providing only subtle ambient sounds, not BAM, BOOM, CRASH. Older films, created before Atmos existed, will not provide much of a 3D effect.

If the listener is well placed with respect to the bar and the room is not mucking with things too much -- and the source material is cooperating -- the rear speakers are optional. In typical living spaces, with typical source material and playback equipment, rear speakers are a big plus.

In controlled situations interesting things can happen. I was at trade show and a representative approached me from the rear and introduced himself. I turned to face him and there was no one there.