How to give Airplay traffic priority on network?
I don't know how to fix your issue but i can help you to go around it.
So if you have the wifi 5GHz (dual mode) you can put your playing device on that network and the downloading device on the 2.4GHz network. This somehow helped me to fix the dropouts while watching videos online.

my config is: iPad playing music via airplay audio, macbook Pro downloading a video. airport express with dual wifi.

It is not the final solution neither the solution you are looking for but it helps in some cases.