my_sound.loadSound("music/track1.mp3", false); This tells me that the music is being downloaded and is not actually in the flash movie or library. Look in the folder that you downloaded. It should contain a subfolder named music. Inside (I think) will contain mp3's named track1.mp3,track2.mp3, & track3.mp3. my_sound.onSoundComplete = function() {
numtrack++;
if (numtrack == 4) {
numtrack = 1;
}
This is incrementing the track number by one at the end of the song and resetting the song number after track3.mp3. To change the songs, take the mp3's that you want in the player and name them track1.mp3, track2.mp3...and replace the current songs in the music folder.
This is all just a guess but hopefully it will help get you going in the right direction.