HDMI and Old Gear

I am using the built-in apps on my Sony XBR-55X850C TV for video streaming. The set’s optical digital audio output is connected to an older Denon AVR-4306 receiver. My problem is that I only get Dolby Digital 5.1 sound from the TV’s Netflix app; no other app (HBO Now/Go, Sony Crackle, Hulu, or Disney+) provides DD 5.1. After much internet research, I suspect that it has something to do with either the Denon’s