If you are using the Arc as fronts, the Arc must be wired to the TV via HDMI-ARC or eArc. You cannot wire the Amp to the TV if it is used as surrounds. 

So you cant wire the Arc to the TV using the Audio out port?

You must connect the front device (in you case the Arc) via HDMI- Arc/eArc.  The rear surround device does not need to be connected to the TV at all.

Sorry for the confusion:

If I connect Arc to TV via HDMI Arc, how does the audio signal get to the Amp for the rears?

The front Sonos Amp or sound bar connects to the Sonos Amp driving the surround speakers using a wireless connection, on a 5Ghz channel.

You mean front Sonos ARC?

Sure, a Sonos Arc is a Sonos sound bar. 

Ok just so I totally follow (I’m new to Sonos)

Connect the Sonos ARC to the TV via the TV’s HDMI-ARC port and that’ll be front L/R/C.

Then the Sonos ARC will connect wirelessly to the Sonos AMP which not connected to the TV at all but is hard wired to the passive rear speakers.

Do I have this correct?

You do indeed. It will be pretty straight forward in the app, I suspect you’ll find it easier than expected. 

Awesome, thanks very much!

You’re most welcome. I’m anxiously awaiting my Arc as well, and the software update that includes the arc information. ;)

If you do run into issues, feel free to post back, there are many members of the community who would be happy to help you get through the process. 

I’m building a new house and wont be moving in for a few months so I’ll need to wait until then, but all the pre-wiring is done!

And I assume the wireless capability also works with other Sonos SoundBars (Beam/Playbar etc)?

Yes.

There’s a single oddity which you won’t run in to, the oldest PLAY:5 (gen 1) doesn’t have the 5Ghz antenna necessary to act as surrounds. But that’s mostly a comment for others who read this thread, as you’re building new, and using the Sonos Amp to drive your own surrounds. 

Is 5GHz wifi essential for the AMP and ARC to pair?

It’s what Sonos uses, created by the front device, to ‘bond’ the surrounds. Your router does not need to generate it, it is a ‘hidden’ network created by the sound bar (or Amp) used for connection.
Gah. 2Am, thoughts not organized.

5Ghz channel is created by the front device, either a Sonos sound bar or Sonos Amp, that the surrounds, or Amp powering the surrounds, connect to. 

Ok so it wont matter if my router is running 2.4 or 5 GHz?

Well, the front sound bar or Amp will only connect to 2.4 GHz. There is no need for your router to run a supporting 5Ghz  channel. For Sonos purposes, they create a special network for the Sonos device to connect to, which is called ‘bonding’. The only reason it’s important to be aware it is 5Ghz is for possible interference issues, but you won’t see that channel. Only the Amp, in your case, will. 

You’ve been an amazing help, have a good night!