Mp3 Player progress bar

I have an mp开发者_Python百科3 progress bar with my mp3 player.When the song is loading/playing the progress bar jumps around until the song has completely downloaded/loaded.

Is this the normal functionality.

I am using the following on an EnterFrame Event.

duration = this.duration;
   pos = this.position;
   //The songs postion
   songPosition = (pos/duration)*100;
   updateProgress(songPosition);//Updates the progress Bar

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Position and duration may not be known until the song gets loaded right. So I will initiate the progress bar after loading. (onLoad listerner)