mp3 length before fully downloaded

Hi,

As far as I know, Flash doesn’t know the exact length of an MP3 loaded via URLLoader (or similar) before it’s actually fully downloaded. There are ways to approximate it but they are close to zero precision to me :

lengthMili = (mySound.bytesTotal / (mySound.bytesLoaded / mySound.length)) / 1000;

Can you tell me how a player like hypem.com knows the lenght, then?
HYPEM uses : http://static.hypem.com/h4_skin4.swf