Its easy, imagine your sound has four parts A,B,C,D. So you'll need the sound to perform twice measures like A,B,C,D-A,B,C,D. Let's imagine that each measure takes 10 seconds so the total time is 20 seconds.
Then in your code:
//add a playback method to follow playhead
yourSound.addEventListener(MP3Loader.PLAY_PROGRESS, onProgress);
function onProgress (e:LoaderEvent)
{
if (e.currentTarget.soundTime >= 11) //just after one second of the first measure, take playhead to one second after the beginning of your sound (in my case I have to use .95 to match perfectly)
{
DKK.gotoSoundTime(.95, true);
}
}
there you have a gapless sound using .mp3