Get ID3info

how can i read taginfo from internal mp3? It’s easy if I load external mp3 files, but doesn’t work for mp3 from my library!

CODE:
var mymusic:Sound = new mymp3(); //mymusic = new var and mymp3 the class of mp3 file in library
mymusic.addEventListener(Event.ID3, getID3);
mymusic.play(); //it really plays

function getID3(Event):void{
var showtag:ID3Info = mymusic.id3;

trace(showtag.id3.artist);//nothing happens
}

Thank You
Ademar