Resolved with iOS7
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First thing to do is check the mute switch on the side of your device. 

If this did not fix it, we are aware of an issue with some devices not playing Video Discoverables and Audio Discoverables. This is due to a bug in iOS 6 and will hopefully be resolved by Apple in their next release.

More info https://discussions.apple.com/thread/4322032

***Update*** Until apple solves this in their next release some users are reporting that if you plug in headphones you are able to listen to the audio.