arc ultra

If you read the guide, you’ll find:

”

• If placing Arc Ultra below your TV, keep 4 in (10 cm) of space (minimum) between the top of the soundbar and the bottom of your TV.       ”

Since your media wall is a “custom media wall”,  you should be able to incorporate the space quite easily. 
 

If you don’t want the upfiring speakers, stick with the standard Beam. 

See https://www.sonos.com/en-us/guides/arcultra

Wil the fireplace not heat up the Arc Ultra?

I see a win/win here. The soundbar will sound better if you place it on top of mantle and your wife will have no place to clutter under the TV. All fun aside though - your TV might also be place pd a bit to high to really enjoy it.

Well, the fireplace is an LED light and only provides heating I turn on the blower.  Not thinking it’ll heat much above the fireplace and the Xmas wreath and holiday decorations are a must, so I’m placing it under.  Basically, will blocking to upwards firing atmos speakers have a negative sound? Does it becomes a front facing soundbar? For the extra $400 bucks doesn’t sound that much better than the beam ultra?  I don’t need the Dolby, it is just a waste of money?  The arc ultra looks pretty bad ass mounted to the wall under the mantel.  

In short… Yes!

Oddly enough placing a solid object infront of any speaker, or placing the speaker behind a solid object, will negatively impact the sound as you are blocking the speaker.

In this case you will totally obliterate any dolby atmos surround and definitely compromise the ultras general performance...

You need a forward firing only soundbar for that position at best as the overhang will also restrict forward and side firing speakers impacting spatial openness and directing sound downwards. 

Not an ideal position for any soundbar really!

One more? That is not a good place for a sonos soundbar. Unlike a home theater setup using discrete components, Sonos’ soundbars are designed to reflect & bounce sound off surfaces to give you your home theater experience. That placement doesn’t really work. However, on top of the mantel...

Yes, paying a premium for an Atmos soundbar and then purposefully blocking the Atmos speakers is a waste of money.  

Neither the Arc Ultra nor the Beam Ultra will work in the arrangement you have shown. The only option for those soundbars is to raise the TV (which shouldn’t be lot if the rendering shown is near accurate) and place the sound bar on the mantel. 

The alternative soundbars are the Beam 2 or Ray which will work below the mantle. The Beam2 does have side firing drivers for a simulated surround (the Ray I don’t recommend).

FYI, while the Arc Ultra and Beam Ultra offer decent surround as standalone speakers; the Beam2 will definitely benefit using Era 300’s or Era 100’s as rear surrounds.

Why not mount the TV a bit higher and mount the Arc Ultra to the TV's mount?

Atmos works, note you aren't blocking the up-firing speakers, just redirecting their sound.

Trueplay works

Removes heat from the problems list.

Leaves the mantel for the spouse's use.

If it is a bit high for your taste look at the "Mantel Mount" that will allow it to move lower..

Regardless of the soundbar being placed above or below the mantle, the soundbar should be brought forward to the front edge of the mantle. Since you will not be using DOLBY Atmos, vertical spacing for a soundbar mounted under the mantle is not critical — if the soundbar is brought forward.

Are you installing surround speakers?

Hopefully your TV also includes an analog audio output and your hearing aids support Bluetooth. If so, you can install an audio adapter to send Bluetooth audio to your hearing aids. Make sure that the Bluetooth arrangement includes a Volume control. There may be other means to send TV audio to the hearing aids, but we’d need more information about your TV and expectations.

Trueplay is not very effective compensating for poor soundbar mantle mounting.

Many viewers will be uncomfortable while sitting and viewing a TV mounted above a fireplace. If this is not your situation, moving the TV high enough to mount the soundbar under the TV (with proper spacing) might be an option.

Mounting the TV and soundbar on a pull-out dropdown mount is another option. Make sure that this arrangement clears your wife’s trinkets on the mantle.

Since yours is a work in progress; you might consider having your builder place a shelf below the mantel to accommodate the Arc Ultra or Beam Ultra as shown below. In my drawing I have decreased the distance (up/down) between the Arc Ultra and the front of the Mantel which you can see in the cross-section view.

However, decreasing the distance is a personal choice and not necessary. In fact leaving the distance as shown in the rendering allows for more flexibility to run power and HDMI to TV. In any event allow enough space between the shelf and bottom of mantel for HDMI cable and power line runs.
 

Or use a wall-mount attached to the front of the mantle, that would look pretty sharp.

I don’t think the wife wants to drill holes in the mantel 😂

Pull down mounts with soundbar attachments?

I tried to post these twice, including the links. I guess the forum software didn’t like it. Just search for these.

MM700 Premier Pull Down TV Mount
MM815 Motorized Drop Down & Swivel TV Mount

Properly done (threaded insert) the mounting holes can be quite small. I had a mantle many years ago and used them. I had several different things mounted on decorative hooks, to include plants, Christmas socks and other decorations. When not in use I had unobtrusive covers.