Usually, a TV set will have one ARC/eARC port. Your Sonos should be connected to that. Your Denon shouldn’t be part of the equation, you should be using one, or the other, but you should not be able to use both. 

Are you using your receiver as a (glorified) HDMI switch? The eARC port on the receiver might be seen as an in to the receiver, not an out. Did you try another HDMI connection?

Ah, good point, an HDMI switch device (of any type) could indeed be connected to any HDMI input. It’s only the Sonos that should be connected to the ARC/eARC output.

Thanks. I’ll check out the receiver specs. Was also thinking I could put an HDMI switch between the ARC and the TV and Receiver. This way I can flip between the TV and Receiver.