The Arc needs to be connected to a TV or projector via HDMI ARC/eARC or optical. I don’t believe it will work if it is connected to an AV receiver.

Read through this thread:

The receiver won’t create an ARC signal, it will just pass one through. Does the Epson Home Connect 4010 create an ARC signal to pass back to the Yamaha, which would then pass it through to the Sonos Arc? I’d certainly agree, the Sonos Arc won’t work being connected to a receiver without an ARC (or eARC) signal. Sonos doesn’t, on any of their devices, interpret a standard HDMI signal, it must be ARC. 

The Epson does not make an eARC signal. The Sonos Arc has a HDMI for the input/output into the arc, is their a convertor that we will need? Sorry I am just lost.

The Arc requires an ARC or eARC signal.

Most often, this is created by the TV set, although there are some rare projectors that can do this. Sonos does make an adapter that will take an optical signal and create an ARC signal, assuming one of your devices can output a digital optical signal, either stereo or Dolby Digital. You could potentially extract the optical signal before it reaches your projector by using any number of HDMI switches available, that also have an optical output. Search Amazon, pick which one meets your needs via number of inputs, just be sure they have an optical output for audio. 

If you want to try to get an Atmos signal to the Arc, you’d need to go with a device like the HD Fury Arcana, which can take a standard HDMI input and output an Atmos signal before the video reaches the projector. I’ve heard rumor of another device that can do this, but I’m not familiar with a name to provide a link. 

There are dozens of threads in this very area of this forum that go over this at length.