music = new Sound();
music.attachSound("SOUND HERE"); // MUST give the sound file a name for AS
music.start(0, 999999);

vol = 0;
step = 1; // Amount to inc/dec the volume by each step
fade = 0; // Do nothing

_root.onEnterFrame = function() {
        // do fade out
        if (fade == 2) {
                vol = vol-step;
                if (vol<0) {
                        vol = 0;
                        fade = 0;
                }
                music.setVolume(vol);
        // do fade in
        } else if (fade == 1) {
                vol = vol+step;
                if (vol>100) {
                        vol = 100;
                        fade = 0;
                }
                music.setVolume(vol);
        }
};