The problem is not with the player but with Firefox and Internet Explorer. I get the same problem of files not loading using Media Element and Videojs, where the player is rendered in the browser but the media file doesn't load. Chrome seems to be the only browser which plays the MP4. I'm using IE 9 and Firefox 12 which are not cooperating with the player.
The following link on the Mozilla site may be of help to the adventurous: https://developer.mozilla.org/en/Using_HTML5_audio_and_video "Since Firefox doesn't support MP4 and 3GP due to their patent-encumbered nature, the <source> elements with the IDs "mp4_src" and "3gp_src" will receive error events before the Ogg resource is loaded. The sources are tried in the order in which they appear, and once one loads successfully, the remaining sources aren't tried at all."