mp4 videos not playing on Internet Explorer 9

Question

mp4 video is not playing and its showing red cross in Internet Explorer 9. It is working fine in chrome

Problem is similar to question posted here: HTML5 video plays in chrome but not IE9

But the solution is not clear

Please help! Thanks in advance

CODE:

 <video width="520" height="540" controls="true" id="mp4Vid" >
        Your browser does not support the video tag.
        <param name="allowfullscreen" value="true">  
        <param name="allowscriptaccess" value="always">  
        <param name="flashvars" value="file=/mov/scene.mp4">  
        <source type="video/mp4" id="source" />
    </video>

Source for video is loaded using following javascript code

 var video1 = document.getElementById('mp4Vid');
 var source = document.getElementById('source');
        source.setAttribute('src', "myMp4.mp4");
        source.setAttribute('type', 'video/mp4');
        video1.pause();
        video1.appendChild(source);
        video1.load();
        video1.play();