Airplay audio out

Not strictly an infuse issue, but figured someone might have looked into this…

Have tried outputting audio from my atv4 to speakers via an airport express.
Unfortunately the connection reverts back to output via internal speakers at regular intervals.
Means i have to go back to the audio settings almost every time i turn on the atv and sometimes even between episodes / films.

Anyone have this working?
I have a solid network, the airplay & speakers works flawlessly in all other respects. Somehow tvOS is just poorly adapted to regular audio output via airplay.