var audioContext, audioProcess, audioSource,
    result = document.createElement('h3'),
    output = document.createElement('span'),
    mp3 = '//www.jonathancoulton.com/wp-content/uploads/encodes/Smoking_Monkey/mp3/09_First_of_May_mp3_3a69021.mp3',
    ogg = '//upload.wikimedia.org/wikipedia/en/4/45/ACDC_-_Back_In_Black-sample.ogg',
    gotData = false, data, audio = new Audio();
 
function connect() {
  audioContext = window.AudioContext ? new AudioContext() : new webkitAudioContext(),
  audioSource  = audioContext.createMediaElementSource( audio ),
  audioScript  = audioContext.createScriptProcessor( 2048 );
 
  audioSource.connect( audioScript );
  audioSource.connect( audioContext.destination );
  audioScript.connect( audioContext.destination );
  audioScript.addEventListener('audioprocess', function(e){
    if ((data = e.inputBuffer.getChannelData(0)[0]*3)) {
      output.innerHTML = Math.abs(data).toFixed(3);
      if (!gotData) gotData = true;
    }
  }, false);
}
 
(function setup(){
  audio.volume = 1/3;
  audio.controls = true;
  audio.autoplay = true;
  audio.src = audio.canPlayType('audio/mpeg') ? mp3 : ogg;
  audio.addEventListener('canplay', connect);
  result.innerHTML = 'Channel Data: ';
  output.innerHTML = '0.000';
  document.body.appendChild(result).appendChild(output);
  document.body.appendChild(audio);
})();