ActionScript 3 Flash CS6-淡出音频

我有菜单上播放的背景音乐。每当玩家选择"启动"按钮（startBtn）时，我想使其顺利淡出。你是怎样做的？这是我的代码：

import flash.events.Event;
import flash.events.MouseEvent;
import flash.system.fscommand;
import flash.media.Sound;
import flash.media.SoundChannel;
import flash.net.URLRequest;
import flash.events.Event;
import flash.media.SoundTransform;
import flash.events.MouseEvent;
//Declare MUSIC Variables
var musicSelection:Sound;
var musicChannel:SoundChannel;
var musicTrack:String = "music/dearly_beloved.mp3";
var musicVolume:Number=0.2;
/*var isMuted = false;*/
loadMusic();
function loadMusic():void 
{
    //first stop old sound playing
    SoundMixer.stopAll();
    musicSelection = new Sound();
    musicChannel = new SoundChannel();
    //create and load the required soun
    musicSelection.load(new URLRequest(musicTrack));
    //when loaded - play it
    musicSelection.addEventListener(Event.COMPLETE, musicLoaded);
}
function musicLoaded(evt:Event):void
{
    //finished with this listener
    musicSelection.removeEventListener(Event.COMPLETE, musicLoaded);
    //play music
    playMusic();
}
function playMusic():void
{
    //play the random or selected music
    musicChannel = musicSelection.play();
    //setting the volume control property to the music channel
    musicChannel.soundTransform = new SoundTransform(musicVolume, 0);
    //but add this one to make repeats
    musicChannel.addEventListener(Event.SOUND_COMPLETE, playAgain);
}
function playAgain(evt:Event):void
{
    // remove this listener and repeat
    musicChannel.removeEventListener(Event.SOUND_COMPLETE, playAgain);
    playMusic();
}

这是第二帧的代码：

import flash.events.Event;
import flash.media.SoundTransform;
stop();
optionBtn.addEventListener(MouseEvent.CLICK, goToOption)
function goToOption(event:MouseEvent):void
{
    nextFrame();
}
startBtn.addEventListener(MouseEvent.MOUSE_DOWN, stopMusic);
function stopMusic(event:MouseEvent):void
{
    musicChannel.soundTransform = new SoundTransform(musicVolume, 0);
    while (musicVolume > 0)
    {
        musicVolume--;
    }
}
quitBtn.addEventListener(MouseEvent.MOUSE_DOWN, closeGame);
function closeGame(event:MouseEvent):void 
{
    fscommand("quit");
}