[Nata_cher]

а просто Sound.duration();
вот пример из хелпа:

var my_sound:Sound = new Sound();
my_sound.onLoad = function(success:Boolean) {
    var totalSeconds:Number = this.duration/1000;
    trace(this.duration+" ms ("+Math.round(totalSeconds)+" seconds)");
    var minutes:Number = Math.floor(totalSeconds/60);
    var seconds = Math.floor(totalSeconds)%60;
    if (seconds<10) {
    seconds = "0"+seconds;
    }
    trace(minutes+":"+seconds);
};
my_sound.loadSound("song1.mp3", true);