The automatic transcription of YouTube vidoes is pretty amazing, and sometimes histerical at times.
However, if for some reason you want to get a raw copy of the automatic (and possibly manual) youtube captions, you simply use the TTS_URL key of the yt and append some magic URL parameters: &kind=asr&fmt=srv1&lang=en.
The following JavaScript will automatically redirect you to the xml file of the transcription:
if(yt.config_.TTS_URL.length) window.location.href=yt.config_.TTS_URL+"&kind=asr&fmt=srv1&lang=en"