title: Issue with getting duration attribute of an audio file when cache is empty
body: <p>I have a problem with getting the metadata of an audio/mpeg (mp3).</p>
  
  <p>For example, I'm using JS to get the duration of the audio file and when the cache is empty the duration value returns "Infinity" (NaN).</p>
  
  <p>I have tried with the events/attributes preload and onloadedmetadata and always when the cache is empty I can't get the duration and other properties of an audio.</p>
  
  <p>Note: The problem is only when I clean the cache (or when the visitors comes first time to the page).</p>
  
  <pre><code>    audioElement = new Audio('http://www.html5rocks.com/en/tutorials/audio/quick/test.mp3');
      console.log(audioElement);
      audioElement.addEventListener("loadedmetadata", function(_event) {
      var duration = audioElement.duration;
      console.log( duration );
      });
  </code></pre>