@constant kAudioSessionProperty_AudioInputAvailable
A UInt32 with a value other than zero when audio input is available.
Use this property, rather than the device model, to determine if audio input is available.
A listener will notify you when audio input becomes available. For instance, when a headset is attached
to the second generation iPod Touch, audio input becomes available via the wired microphone.
では、kAudioSessionProperty_AudioInputAvailable に関する通知を受け取りたい場合は、どうすればよいでしょうか?