org.osmf.media.MediaPlayer Throw Error: The specified capability is not currently supported

I have a back end sound.php which can return .m4a sound file from web server and I can make a web request with id to the sound.php to return specify .m4a file. i.e. sound.php?id=1234

I am now trying to use   org.osmf.media.MediaPlayer  and AudioElement  and URLResource

  var mediaPlayer:MediaPlayer = new MediaPlayer();  var ae:AudioElement = new AudioElement(new URLResource("http://xxx.com/sound.php?id=12"));  mediaPlayer.media = ae;  mediaPlayer.play();

and it throw error of The specified capability is not currently supported .I have tested the link via browser which is return a .m4a file sucessfully .

I dont understand if it is claiming the requesting method or the returned file , would somebody has any idea? Thanks