I recently set up Airplay on my Raspberry Pi 3 using shairport-sync and Audioengine A2+ speakers, connected via USB.
When I try streaming anything from Vox, audio is interrupted almost every second. However, when I try streaming the same files from iTunes, audio playback is flawless. This is the case for both compressed AAC and lossless ALAC. The only difference is the program used for audio playback; the hardware and the network setup are exactly the same.
My Macbook Pro runs OS X El Capitan 10.11.5. Maybe this is an El Capitan issue.
Just as an aside, Airplay works fine on all iOS 9 devices (iPhone & iPad) can stream fine via Airplay. On my rooted Android device running Allstream, however, the same interruption happens. Getting Vox running is my top priority, but I thought this information might be useful to some.
David