Q: AirPlay - No sound

Hello,

I've conneted a 1. gen AirPort Express to my router via a cat6 cable and the audio cable to my hifi. I have a 20/5 mbit internet connection.

When I play music via iTunes on my mid-2009 MacBook Pro with Yosemite wireless, there are no problems streaming it via AirPlay and the AirPort to my hifi.

When I try to stream music from my iPhone 6 or my iPad Air, both updated to the latest version of iOS, no sound comes out of the hifi.

I've hard reset the AirPort and also turned off wireless network in the AirPort, since it is connected to the router via the cat6 cable.

What can I do to fix this annoying problem?

Thank's in advance :-)