My Denon 3312 is connected to my network via Ethernet. I set the Source On my Denon to Network (instead of BD, DVD, TV, etc).
On my iPad's Music, there is a drop screen on the upper right corner. Once tapped, it shows two options for music play: "iPad" and "Denon AVR-3312".
I tap the Denon. Then tap Play on my iPad. The music plays on my Denon/speakers.