title: How can I get the html5 audio's duration time
body: <p>I have a html5 <code>&lt;audio&gt;</code> tag in page, but how can I know its duration time?</p>
  
  <pre><code>&lt;audio controls=""&gt;
      &lt;source src="p4.2.mp3"&gt;
  &lt;/audio&gt;
  </code></pre>