Why track sounds speed up?

hi everybody,
Im new to this and my problem is that im setting up a mp3 player for my site …everything work fine loading the track when i click the track 1 bottom and i listen to the .mp3 file in its original bitrate at 320kbps but if i convert it at a lower bit rate and play the track it sounds very speed up .Im a bit retard with flash yet so if anybody could help me will be fantastic.
thank you

PS:…this is my AS for the button


var track_1:Sound = new Sound ();
track_1.onLoad = function (success:Boolean):Void {

if (success){

track_1.start( 0 , 1000 );

change_volume ();


track_1.loadSound (“track_1.mp3”, false);