Unfortunately you will probably hear little to no height channel effects from the two upward firing drivers because of your vaulted ceiling and the height of your ceiling.

So do you recommend that I abort and not get the ARC?  How do those that have this situation configure the room to get good sound?

It depends on how important hearing height channel effects from Dolby Atmos content is to you. The Arc will still probably sound great in your room. And Dolby Atmos audio is more than just height channels.

People in your situation who want to experience height channel effects don’t rely on speakers or sound bars with upward firing drivers. They have a more traditional home theater setup with actual speakers installed in the ceiling.

If adamant you want Atmos audio then. personally speaking. I wouldn’t choose the Arc with upward firing speakers aimed at a vaulted ceiling. You might be better off with the new Beam (Gen2) instead, as that has no upward firing speakers, but creates a ‘virtual’ atmos audio effect by using built-in processing trickery. I would opt for that if you really want a Sonos setup and the room is not huge in size and could cope with that size of Soundbar, surround and one or two Subs.
 

Alternatively another option is to step outside of the Sonos idea and install in-ceiling speakers ‘angled’ and down-firing for Dolby Atmos, but wired to a HT Receiver, but before doing that I’d recommend talking to a home theater installation expert for options involving vaulted ceilings.

I’m not sold on getting perfect Dolby Atmos but I do want to have a great sound in the room.   can certainly add speakers to the ceiling if that will make it better.  That said, the ceiling is vaulted and it starts at 12’ so the speakers would be mounted at probably 14-16 feet up.  Will that solve the problem if I do that, add front bookshelf speakers and just eliminate the ARC?

You can always test the Arc in your room and see how it sounds. If you don’t like it, return it and find a different solution. I would recommend adding the Sonos Sub and two One SLs as surround speakers too.

As @Ken_Griffiths mentioned, you might consider having a home theater installer take a look at your floor plans so they can recommend the best options for your particular room.

Thanks for your help and advice Ken.  I’m still not sure what to do but it does make sense to get a HT specialist out to help design it.   

Ceiling installed downward firing speakers (for the purpose of the Atmos-height audio channels) unfortunately cannot be bonded to any Sonos HT setup, so you would have to look at a non-Sonos Receiver - but if your not too bothered about the Atmos audio experience, then I think the ‘Arc’ or the ‘Amp’ still sound great anyway - as does the new Beam for it’s size - you can just switch the connected TV/Devices to output Dolby Digital 5.1 audio instead, which on the Arc sounds really good to my ears.

This is the layout of the room.  It’s basically a 20’ x 20’ greatroom.  I was planning to put a sub in there as well but I’m not sure how the vaulted high ceiling will affect that.

I added the ceiling speakers to the drawing just to see if that will work in conjunction with the ARC but… after listening to you guys, I’m just not sure what’s the best config for this room. It’s still all open framing right now so if I need to add the ceiling speakers, I think I should do it now.

@Bearstone,
Ceiling speakers bonded to a Sonos HT are connected via the Sonos Amp, but that’s often just for rear (surround) audio-channels only. 

Personally speaking, I don’t like the two rear TV channels coming from above my head. I much prefer the rear channels at head height level when seated. I prefer overhead speakers just for background music listening.

I think I would like the ceiling speakers though, for Atmos ‘height’ channel audio, but as mentioned that’s not currently possible with a Sonos HT setup and not that my wife would let me put speakers in the living room ceiling here anyway.

So my thoughts are it’s still a good idea to bring in a HT installer to see what suggestions they can bring to the table, but I don’t think you would be disappointed with the Arc, Sub and Surrounds, even with the vaulted ceiling. 

Note, as mentioned earlier, there is a Sonos returns policy too (45 days, I believe) if the devices are purchased direct from Sonos, as outlined here in this link:

https://www.sonos.com/en-us/legal/terms?#legal-terms-sale-returns

So it provides an opportunity to maybe test the Arc HT setup in your room too.

If you really need the height channels I’d suggest a high quality AVR and a matched set of individual speakers. Your local home theater specialist is likely your best first stop for suggestions.

There are some decent in-wall solutions but you’d really want to verify how much sound will leak out the other side of the wall. Surface or arm mounted speakers are less likely to intrude on the other rooms.

Pick up a Sonos Port to feed all your Sonos music into it and integrate with the rest of your Sonos systems.