HDMI Output to Denon AVR-X2000

For some reason I am unable to play audio over HDMI from a RPi 4 to my Denon AVR-X2000.

I also have a later model of Denon, (AVR-X2300), where this works with no issue. I have two identically configured RPis and get the same results with both, irrespective of HDMI input used.

Any ideas?