I have a new Raspberry Pi 3B+ and HiFiBerry Digi+ running the 20181228 release of HiFiBerryOS. It is connected via ethernet cable to my router, and I've connected the Digi S/PDIF to my DAC. iTunes finds it as an airplay receiver, and all of the expected LEDs flash like data is being sent when music is played but no sound comes out of my amplifier.
Running aplay -l yields:
**** List of PLAYBACK Hardware Devices ****
card 0: sndrpihifiberry [snd_rpi_hifiberry_digi], device 0: HiFiBerry Digi+ Pro HiFi wm8804-spdif-0 
Subdevice #0: subdevice #0
alsamixer confirms that the Digi Pro is recognized and the volumes are not muted. Where should I look next?