test.js

var video = document.getElementsByTagName('video')[0];
var audio = new Audio();

function saveMeta(event) {
  audio.mozSetup(video.mozChannels, video.mozSampleRate);

}

function saveAudio(event) {
   audio.mozWriteAudio(event.frameBuffer);

   console.log(audio.duration, video.duration);
}

video.addEventListener('MozAudioAvailable', saveAudio, false);
video.addEventListener('loadedmetadata', saveMeta, false);