var audioContext, audioProcess, audioSource,

    result = document.createElement('h3'),

    output = document.createElement('span'),

    mp3 = 'http://songs.jonathancoulton.com/mp3/First%20of%20May.mp3',

    ogg = 'http://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);

})();