With that receiver, there may be one way to get audio to the Port, and that would be through an audio extractor. The only outputs are either speakers, Subwoofer or the single ARC.

There are a few inputs that are able to play whatever is streamed from Sonos to the Denon, but not the other way around.

Thanks for the reply. Not exactly was I was hoping to hear. Would an audio extractor negatively impact sound quality?

Have you activated the digital coax in setting in the Denon receiver? 

The coaxial connection on the Port is an “out”, not an “in”. The coaxial connection on your receiver is an “in” not an “out”.

​@BobS208 

Indeed you can use a simple hdmi audioextractor that looks like this one…

But I doubt the build in DA Converter will be of high quality. 
So I would test it and probably use a separate DA converter. 

​@Schlumpf OP wants sound from his receiver to the Port. If I understand your solution right, the converter would covert a signal from the HDMI out on the receiver to a signal the Port can handle. The only “in” on the Port is an analogue (RCA) in, if I remember correctly. So OP would also need a digital to analogue decoder to get sound from the receiver into the Sonos system?

The device I posted pictures of above is a simple hdmi audioextractor and could do that job as it has three output options. One of them is analogue stereo output. 
…HDMI out of Receiver to HDMI in of audioextractor and analogue out of audioextractor to analogue in of Sonos Port…
But imo the DAC quality of these cheap devices probably is not the best. So I would try it and, if not satisfied, possibly use another separate high quality DAC that is connected via optical or coax digital to the audioextractor and then probably will do a better job on DA conversion.

That high quality analogue signal can be used on Ports line in. 
Not a design winner, but maybe the best signal quality.

PS: 

An example (about 90€)…

Unfortunately I didn’t find any „all in one“ DAC device that also supports a HDMI input.