How is this class connecting to the sound? - actionscript-3

Question

im trying to use (well succeeding) to use this sound class
http://www.mcfunkypants.com/2011/as3-pitch-shift-mp3/
the example code looks like this. . .
public class Pitch_Shift_Example extends Sprite
{
[Embed(source='Pitch_Shift_Example.mp3')]
private var engine_mp3 : Class;
public var engine_loop:Pitch_Shift_MP3;
public function Pitch_Shift_Example()
{
engine_loop = new Pitch_Shift_MP3(engine_mp3);
stage.addEventListener(MouseEvent.MOUSE_MOVE, onMouseMove);
var someText:TextField = new TextField();
someText.x = 200;
someText.y = 0;
someText.textColor = 0xFFFFFF;
someText.selectable = false;
someText.autoSize = TextFieldAutoSize.LEFT;
someText.text = "Pitch Shift MP3 Demo by Breakdance McFunkypants\nMove your mouse to change the sample rate. Enjoy!";
addChild(someText);
}
private function onMouseMove(event:MouseEvent):void
{
engine_loop.rate = (mouseX / stage.width) * 2;
}
}
Now what is confusing me is how the engine_mp3 class uses the sound.
line one embeds the sound.
line two creates an empty class called engine_mp3.
line 3 creates a new pitch shift class which in line 7 we pass the (empty) engine_mp3 class.
Basically how is engine_mp3 getting the mp3 data??
Any help is appreciated.
Andy

Answer 1

It works like something like this:
You embed the mp3 file with these two lines:
Embed(source='Pitch_Shift_Example.mp3')]
private var engine_mp3 : Class;
Those two lines work together. The first line is embed metadata that describes the line below it. What it's doing is embedding the mp3 file and making it accessible as a class. You therefore have a reference to this embedded sound class with the variable name engine_mp3.
Later on in your code, you instantiate a new instance of the Pitch_Shift_MP3 class, and you pass in the reference to your embedded sound class engine_mp3:
engine_loop = new Pitch_Shift_MP3(engine_mp3);
The constructor for the pitch shift class is expecting a class (that represents a sound) as its single argument. What it must be doing in its own code is instantiating the class that engine_mp3 represents by doing this:
var instantiatedSound:Sound = new engine_mp3() as Sound;
Hope that makes sense!