Just to say the Beam and Arc contain the front left/right/center TV channels, aswell as the height channels - you cannot add any Amp, or additional speakers, to replace those included channels. You can only add two rear channel speakers and Sub(s) to those soundbars. Neither the Arc, nor the Beam can just become the front center channel only.

Ok, lots of feedback, some of it may not be what you want to hear. 
 

First, your current setup is 5.1, not 3.1. The Beam has front right, front center, and front left in that enclosure, that is 3.0 right there. The Sun is the .1, of course, but the two in-wall speakers powered by an Amp makes it 5.1, assuming the Amp is bonded to the Beam for surround duty. 
 

Now to your desire…Sonos does not make any system that allows you to feed a center speaker by itself. You either get the center speaker inside the Beam or Arc, or you get a ‘generated’ center speaker from an Amp that is feeding front right and front left. 
 

The Sonos Port can not be used to feed a surround signal at any point, either with a Sonos front end, or with a third party front end. If you feed a Port a surround signal from another device, it will be subject to a minimum delay of 75ms , and not be effective. Sonos, along with every other home theater system I’ve seen, does not want to allow a mix and match situation. 

How does your current setup work? Is the Beam being used as a center channel speaker with two front in-wall speakers and a Sub? Are you grouping the Amp with the Beam/Sub room? If so, do you experience an audio delay from the in-wall speakers?

When using the Amp for a Sonos home theater setup, there isn’t a center channel speaker. Typically you see three options:

1. One Amp powers two front passive speakers. The Amp is bonded with a Sub for a 2.1 setup using a phantom center channel. You can also use a third-party subwoofer instead of the Sub in this setup.
2. 2.1 setup (with Amp) + a second Amp powering two surround speakers for a 4.1 setup
3. 2.1 setup (with Amp) + a pair of Sonos speakers used as surrounds for a 4.1 setup 

If you want to use three in-wall speakers for a 3.1 setup, you need to use a third-party receiver and third-party subwoofer. Then add the Port to the receiver to include the 3.1 setup in your Sonos ecosystem.

Also note a Sonos Port cannot be used as part of any Home Theatre setup - it’s audio output has a 75ms minimum (group buffering) delay and so when playing TV audio, that would cause an echo with any other Sonos Home Theatre products situated in the same physical room .

For a ‘cleaner look’, given your restrictions (spousal approval), I’d probably go with a Sonos Amp driving two front in wall speakers. They’d generate the ‘faux’ center channel, and you could keep the Sub bonded to the Amp. If you wanted, you could get a second Amp, bond it to the first, and drive in wall speakers as surrounds. 
 

That gives you four in wall (hidden) speakers, with just the Sub to explain.  And essentially a 5.1 system (technically 4.1, but I’ve never seen any complaints about the ‘faux’ center channel from people).

Thanks all, yes I didn’t realize that the Beam left/right/center rather than just center.   That makes this more difficult.  
 

To answer GuitarSuperstars question and clarify the existing setup, yes I currently have what I now know to be a 5.1 setup, with 3.0 in the beam and .1 from the sub and the additional two powered from a grouped Sonos Amp that powers the two in wall speakers.   Those speakers are in front, not behind.   There is not a delay, it sounds seamless but maybe I’m hearing what is intended to be sounds behind me (surround) from the front?  
 

Thanks Ken Griffins for explaining that I can’t really do a port setup.  
 

I guess reading from all the options, I have the following three choices:

1.) If I want an unpowered in-wall center speaker, my understanding is Sonos has zero options so I would need to abandon Sonos altogether and get a third party receiver

2.).  Keep the soundbar, but maybe have a custom in wall cubby created (seems silly for a soundbar)

3.).    Abandon the center speaker and have the AMP just power the two in wall speakers and have a  2.1 setup.  

Is it me or is this a really disappointing set of options?     

You state: “To answer GuitarSuperstars question and clarify the existing setup, yes I currently have what I now know to be a 5.1 setup, with 3.0 in the beam and .1 from the sub and the additional two powered from a grouped Sonos Amp that powers the two in wall speakers.   Those speakers are in front, not behind.   There is not a delay, it sounds seamless but maybe I’m hearing what is intended to be sounds behind me (surround) from the front? 

In Sonos speak there's a difference between “grouping”, “pairing” and “bonding". You can group speakers that are in different "room’s” in the Sonos app (see https://support.sonos.com/article/group-and-ungroup-rooms), you can áir two of the same model speakers to a stereo set (at which point they will form one room) or you can bond certain combinations of speakers to a surround set ((at which point they also will form one room).

So the question is: is the Amp grouped with the Beam, which would cause 75ms of delay to the Amp, or bonded to the Beam, wich would mean you are getting sound from the Amp that is meant as rear sound effects, but your speakers are placed wrong.

To your questions:

1.  Yes, you are right.
2. I would not put the Beam in a cubby. The Beam sends it's sound not only from the front, but also from the sides. This could be hampered by putting it in a cubby - as would Atmos if your Beam would be a Gen 2.
3. That would be the only Sonos set up that is possible if you do not want to use a soundbar, yes. I would be very simple set up too. When making place for the speakers take note of Sonos’ tips for how far the spekaers shoudl be apart.

Why would this be disappointing? I think the Amp, that generates a “phantom”  center, is perfect for people not wanting a lot of speakers (the cleaner look you are after, so to say).

Did anyone try a Onkyo, Pioneer or Integra surround receiver with a Sonos port?

As I read the manuals the receiver can be controlled by Sonos. resulting in a tight integration.

Receiver connected to you TV you should have best of both worlds.

I will give it a try.

I used to use an AVR for TV sound and fed it with a Zone Player (early Connect / Port) and it worked wonderfully.

The AVR offered a lot of options Sonos doesn’t that were fun to play with, and one I still miss, the ability to adjust the center / dialog speaker level up a bit.

@dB_1 These receivers can be used with a Port, that’s as far as the Sonos integration goes. Is that what you expect?