where to place arc ultra

Why is there no space below the tv? Depending on the answer there might be a solution. Maybe a picture?

Upside down is very bad, right-left will be a problem, the height speakers even worse.

Right-side up above the TV isn’t horrible but low mounting should be fully explored first.

If you have no way to place a soundbar below a TV, I wouldn't use a soundbar.  Especially an Atmos soundbar which requires a distance from the ceiling sufficient to bounce the Atmos effects off the ceiling and back into into the room to reach the viewing area.  A soundbar mounted near the ceiling will end up with the Atmos effects landing a few feet in front of the screen.

I thought I posted the photo before. 
 

if no soundbar then what speakers should I use near the tv? I have two era 300s for behind the couch. A subwoofer. Should I get more 300s for the tv side wall? 
 

I’m hoping the soundbar above the tv isn’t terrible but sounds like it won’t be great. 

If you can raise the TV a bit a Sanus mount shown in the link may work. 

https://www.amazon.com/SANUS-Soundbar-Mount-Designed-Sonos/dp/B08KJDQBSW/ref=mp_s_a_1_1

The only other option (IMO) are 3rd party outboard speakers on stands powered by a Sonos Amp. However they will be lower than the TV. Although the lower position may work to your advantage to place the speakers at ear level 🤔.

You can still use Era 300’s as surrounds and a Sonos sub. However, there will be no Dolby Atmos sound as with the Arc Ultra. The center channel will be a Phantom channel created by the outboard speakers. * Click the link. 

Others may suggest alternatives but I’m not going down that rabbit hole.

https://www.sonos.com/en-us/shop/amp

Mounting the Arc Ultra above the TV can be done but as your research shows is not ideal.

 * If using 3rd party speakers with a Sonos Amp an alternative to Era 300’s as surrounds would be Era 100’s as IMO they are a better match. The Era 300’s could be repurposed as a dedicated stereo pair for music.

Also, sending TV audio to speakers not part of the home theater setup is not recommended as there will be a 75ms delay causing an echo effect. 

How much space between the fireplace mantle and TV? Could the soundbar sit on the mantle in front of the TV?

The reason I don’t put the tv up higher is because it’s already up pretty high in terms of neck strain. My thought was a sound bar above the tv was the lesser of two evils. 
 

There’s only about two inches btwn the mantel and tv right now. 

I think the distance using the Sanus mount could be negligible as long as you keep just a minimum space  (.5 inches) between the bottom of the Arc Ultra and mantel. If it proves to be to much you can always lower it again without exposing any holes.

The extra height from raising the tv would be hard to notice, you could experiment before committing to a purchase to confirm that. The Arc needs under 3 inches if placed ahead of the TV so that mantle should be deep enough, if not a small extension to the mantle or an extensible wall mount would place it forward enough.

My choice would be to raise the tv, looks to need less than a couple inches and mount the Arc below it. Second choice would be above it and accept the height channels would be a bit off. An Amp and extrnal passive speakers would be a distant third and would be a major step down, no real center channel and no Atmos. That no Atmos includes the 300s which would be a real loss.

OK…

If that fireplace is more aesthetic than functional here’s a solution that will also allow mounting of the Arc Ultra. The link will explain/sell itself (albeit a bit expensive) but worth the investment IMO for the long hall.

https://www.mantelmount.com/products/mm815-motorized-drop-down-swivel-tv-mount

If the ARC can sit on the mantle in front of the TV, the TV would only need to be raised about 1.5 inches.