That’s because it’s playing ‘ambient’ PCM stereo audio and not Dolby 5.1 audio.

So I can’t select hdmi 2 only hdmi 1 tried another cable and no difference 

You will hear quite a huge difference when you get ‘Dolby 5.1’, or better still, ‘Atmos audio’ playing from all your speakers.

You have to switch to the HDMI-2 input to change the setting you’re referring too but, IMHO the ‘Bitstream’ setting you have already is just fine.

But hdmi 2 isn’t showing as being live, that’s what I can’t change the setting?

I still think it’s likely going to be the cable between your TV and the Arc. I know that’s not what you want to hear, but just try another one temporarily and see if it fixes it,

Ideally try the cable that came with your Arc (if practicable).

You would normally switch the HDMI input here by the way using the input selector:

I’ve tried a different cable and it’s the same

if I click on the hdmi input I get this screen

Yes that is the Arc screensaver.

Have you tried using the HDMI cable that came with the Arc?

When you switched HDMI cable and played the 5.1 movie, you still got PCM stereo audio only in the Sonos App ‘about’ box, is that correct?

… and was that from the TV’s built-in Netflix App?

Yes I used the cable the Arc came with and tried Netflix from tv and sky and also Amazon film as well in case it was Netflix 

Could it be the amp?

You could maybe try ‘unbonding’ the Amp from the Arc, and perhaps any Sub too.. and then see what the Arc’s audio-output shows in the App.

These are my LG C9 TV Audio Settings too, FWIW…

• Sound -> HDMI ARC: (Digital Sound Out = Pass-Through / eARC = ‘ON’)
• Sound -> Additional Settings: (DTV Audio = Auto) / HDMI input format = Bitstream for all HDMI ports.
• Connection -> HDMI Device Settings: (Auto Device Detection = ‘ON’ / Simplink = ‘ON’ / Auto Power Sync = ‘ON’)

It could also be the four speakers not matching the output/impedance of the amp.

Once you get the correct output from the TV to the Arc, of course.

Sonos amp and 4 Sonos in ceiling speakers, it has to be compatible. 

What was the audo-in displaying after unbonding the surrounds from the Arc?

Sonos doesn’t make in ceiling speakers, do you mean the Sonance Architectural series of ceiling speakers? Sonos does sell them through the website, due to a marketing agreement with Sonance. But if they aren’t the Architectural series, they may not work as you expect. Once GuitarSuperstar and Ken get you to a point where your TV is passing the correct signal to the Amp, that is. 

Still the same

Well that’s really strange.

Your TV settings, from what you’ve said, seem correct, you’ve swapped cables, tried different Apps that show DD 5.1 audio, so it’s either the TV isn’t working as it should be, or its the Arc.

Can’t say I’ve ever seen this issue here on the forum previously.

My own thoughts, assuming you don’t have easy access to another TV to test the Arc, are to perhaps submit a Sonos diagnostic report, post it’s reference back here and then contact the Sonos Support Staff via this LINK to see if they can diagnose the issue. If not, then it might then be a case of speaking to LG Support who are on this phone number here in the U.K. 0344 847 5454

Thanks for everyone’s help, I’ve reset the TV, the arc, the amp the lot, I have another set up in a different room using a Sonos playbar and it all works fine, don’t understand what the problem is?