Are you using your TV’s internal Prime Video app? Do you get an echo from all stereo sources from your TV’s internal apps?

Yes. Exactly. How enter only on Amazon Prime it seems. Using Apple TV (which forces DD 5.1) I don’t have the issue. 

“I am using 2 Sonos AMP with in-ceiling speakers set up as surround. “

Can you confirm the set up please?  You have two Sonos Amps, one running the front speakers (ceiling speakers?) and the other the surround (ceiling) speakers?

Is there any possibility that when you hear the echo your TV speakers are also playing?

Correct. One AMP has the fronts and the other has the rears. They are both hardwired to the router and sound is send over HDMI-ARC to the first one. 
 

TV speakers are turned off so should not be an issue. The echo also disappears if I switch off surround in the Sonos app. 

Could you try something please? Disconnect the Ethernet from the rear Amp. Power it off and on again then check in About My System that it has WM:0 not WM:1 for both Amps. Does that make any difference?

Further note. In the settings for both Amps is a setting for enable/disable WiFi. This must be left on its default of 'enable'.

It is WM:0 for both Amps in the living room. 
 

I have another one in the bedroom which reads WM:1. Does that mean anything?

It means it is not connecting to SonosNet, which it should be doing.  Have you checked the enable/disable wifi settings on the Amps?  Have you disconnected the Ethernet from the second Amp as suggested?

The reason for my suggestions is that Sonos HT systems are intended to communicate by direct routing over 5GHz wireless.  I am wondering if you have prevented this by wiring the surrounds and turning off the wireless radios.  It is just a possibility.

I prevented this and turn WiFi off. Did this because they are connected to a sub which is located too far from the Amps to connect so I needed that to connect via 2.4

OK I understand. Although they are connected by Ethernet not 2.4. 

I can't really think of any logical reason why this should cause the particular problem you are having. The only way to check would be to experimentally do as I suggest and see if the problem goes away. Whether you think this is worth trying is up to you. 

Thank you very much for the suggestions. Will experiment at but over the weekend and come back. 

Hi @Tejs Dyrvig Ernst 

It may be worth removing the surrounds Amp from the HDMI connected Amp, then adding them again.

This is done by selecting, in the Sonos app, Settings » System »  name of room] » Remove Surrounds.

You can then add the 2nd Amp again in the same app section.

If this doesn’t work, I recommend you get in touch with our technical support team, who have tools at their disposal that will allow them to give you advice specific to your Sonos system - they can check that all your devices are connecting in the way they should be.