Can you tell us if you are using TCP or UDP for the signalling in Zoiper ? The question might sound weird, but iOS behaves differently in background for both methods.
In either case, can you kill zoiper, restart it, keep zoiper in the foreground and test again. Do you have audio now ?
Have you tried changing the rport setting ?
Are you able to take a wireshark packet capture on the wifi network (or on the android phone) to see if there are audio packets present ?