So, a few questions.

Is the Amp ‘bonded’ to the Arc as a surround system, and not just ‘grouped’ with the Arc?

Is the source you’re testing with Dolby Digital, or Dolby Atmos, and not just stereo?

How is that source being fed to the TV? Is it via HDMI or via composite?

When you look at the room name on the ‘settings’ are of the controller, what shows up?

Have you submitted a diagnostic and called Sonos support?

.

I will assume you have successfully ‘bonded’ the Amp/Surrounds to your Sonos Arc (& Sub), via the Arc’s room settings already.

You next need to play a Dolby 5.1 audio source from/via your TV, when testing the TV audio output, it will show you the type of audio on the Arc’s ‘now playing screen’ in the Sonos App. If you’re  not getting surround audio, then check your TV sound-out settings.

To get music audio out to the rear surrounds, I suggest you change the Arc’s "Music Playback’ option to ‘Full’ (rather than ‘Ambient’) in "Settings/System/�ARC Room Name]/Surround Audio"

Do I read correctly that Trueplay says the surroundspeakers are not making sound? Would that not mean the Amp is bonded as surround?

I initially read that as ‘tried playing’ speakers ha ha, but looking at the post again, I think you are perhaps correct @106rallye.

I guess we’ll have to wait to see what the OP says in their next post.

I am an authorized Sonos dealer and installer almost since they started. 
set up countless systems. 
I was playing Master and Commander movie on YouTube and noticed nothing from rears.  
tried the Truplay and got nothing from rear.  Yet the setup was normal.  And it shows a normal set up in app.  
the ARC, TV, AND SUB are in den.  The AMP is in the basement mechanical room in a rack with the other AMPS that power the rest of the house.  The surround AMP is hooked to the router with a cat6 cable. 
I have to go back there tomorrow and work on it some more  

Is the Arc also connected to the router with an Ethernet cable?

YouTube carries stereo, and not Dolby Digital, there would be no action from the rear speakers. Try playing a Dolby Digital source and see if there is some action. If not, then submit a system diagnostic, and call Sonos Support to discuss it.

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution for your client’s system. 

When you speak directly to the phone folks, they have tools at their disposal that will allow them to give you advice specific to your customer’s Sonos system and network.