Greyed out AirPlay symbol in Safari We recently upgraded from Snow Leopard to El Capitan to use the Safari AirPlay feature for embedded HTML5 Videos.

On videos (e.g. YouTube) we see that AirPlay symbol but it is grey and clicking on it shows something like "looking for devices".

The strange thing is that in iTunes everything works fine. We can select our AirPlay speaker and successfully play music from iTunes.

So why can't I stream music from HTML5 videos to my audio system, when iTunes works perfectly?

Network:
Computer is connected via WLAN to an AirPortExpress. The audio system is plugged into the AirPort via this round headphones cable.
You can only stream AirPlay video to a device capable of displaying a video signal.

You can stream audio to a speaker and video to an Apple TV but you can't just pull the audio from the video.