The Arc needs to go through the TV Setup process in the Sonos app while connected to a display that supports CEC before it will work with the Arcana and your projector. If you don’t have a TV with CEC, you may have to take the Arc to a friend or family member’s house to perform the TV Setup process for the Arc.

From the Arcana manual:

I cannot get any sound to a brand new/never used SONOS Arc from any source at Arcana input when Arcana is connected to a non CEC capable projector/monitor or display
In order to get audio from any sources at Arcana input, users of brand new/never used SONOS Arc must go through the APP setup process while connected to a CEC capable display. If you never went through the SONOS APP setup process with your SONOS Arc soundbar yet, please connect it to any HDMI display that offer CEC support and complete the SONOS APP setup process. Once done you can connect it back in your setup with a non CEC capable display, monitor or TV and you will be able to get Full Audio from any source to SONOS Arc as expected. This SONOS restriction requires such initial setup first with CEC capable display. If your SONOS Arc has been setup before, then there is no need to set it up again after Arcana is connected and you can just proceed normally.

Taking it elsewhere shouldn't be necessary. 

In the app setup say you don't have ARC, then skip out when the speaker fails to detect the optical adapter instead.  

You'll need to configure the IR remote and enable Autoplay separately. 

It should then work fine with the HDMI. 

Thanks, but that doesn’t really help me as I don’t know anyone locally who still owns a TV. It seems most people watch Netflix on their laptops these days. 
 

At the moment it seems Sonos and HD Fury are blaming each other for this.
 

If I don’t need CEC to run this, why does the set up need to check for it and how do I disable this check?

Read my post.

I did and I already have it set up that way, but doing so means I can not use Atmos. 
 

How do I get Atmos working without checking for CEC, which is not required for it to work?

Have you contacted the Hd Fury support folks?

Yep that was where I tried first. They blame the Sonos setup for checking for something that is not actually required for Atmos to work. 
 

I don’t understand. If HDMI works, then Atmos should too.

I wasn’t suggesting you use the optical adapter.

I use Arcana, connected to a non-ARC TV, fronted by an HDMI switch. Atmos (from a Fire device) is able to reach the speaker fine. 

Did you plug it into an ARC TV before you used it that way. HD Fury say the ARC is locked until you run it that way for the setup app. Once setup it runs fine. 
 

The problem is I have a projector without CEC, so I’m unable to run the setup with a device that supports it and then plug the Arcana in afterwards