as xela says, you need to use the option in iOS to switch the output to airplay. there is no facility to do this from within the iplayer app AFAICT.

another thing - i have not been impressed with the quality when feeding an analog output from the AE to the hi-fi. i recently started using a Cambridge Audio DacMagic Plus to handle the digital output and the difference is massive. it's very much better than i had hoped and i use it a lot.