Duration streaming mp3

hi :D, just a little question about mp3 streaming… i’m loading a sound in streaming in flash and i would like to do a duration bar but cant do some kind of ratio like duration/position cause the sound isnt fully load when starting to play the mp3 and the duration is updating on evry enterframe… is somebody have an idea plz ? can i just do some kinf of ratio like totalbytes/bytesplayed or somehting like that ? thx