IPhone AudioSessionSetProperty

I got the below code from some sample code. What can cause it to throw a CAXException?

I downloaded the AudioToolbox framework, am I missing something?

Thanks Tee

UInt32 category = kAudioSessionCategory_RecordAudio; XThrowIfError (AudioSessionSetProperty (kAudioSessionProperty_AudioCategory, sizeof (category), & category) "cannot set audio123 category!");