The Script

// create movieclips to hold the sounds
_root.createEmptyMovieClip("empty1", _root.getNextHighestDepth());
_root.createEmptyMovieClip("empty2", _root.getNextHighestDepth());
//create two sound objects
var myAudio_1:Sound = new Sound(empty1);
var myAudio_2:Sound = new Sound(empty2);
//load both sounds and start them
myAudio_1.onLoad = function() {
	this.start();
};
myAudio_1.loadSound("../save_sound/lanier2/track1.mp3", true);
myAudio_2.onLoad = function() {
	//we stop audio 2 for now
	this.stop();
};
myAudio_2.loadSound("instructions.mp3", true);
//
//create 2 variables and give them values according to the volume settings you want
var x:Number = 0;
var y:Number = 100;
_root.onEnterFrame = function() {
	//set volume for sound 2 here
	myAudio_2.setVolume(x);
	//if statement: when sound 1 is half way played, increase volume
	//for sound 2 and decrease volume for sound 1
	if (myAudio_1.position>=myAudio_1.duration/2) {
	   //we now start audio 2
	   if (x == 10) {
	       myAudio_2.start();
	   }
	   //here you can set the fadespeed
	   x += 10;
	   y -= 10;
	   myAudio_1.setVolume(y);
	   if (y<=0) {
	       //stop playing sound 1 now
	       myAudio_1.stop();
	       delete _root.onEnterFrame;
	   }
	}
}