First things first the theory https://gameprogrammingpatterns.com/singleton.html by Bob Nystrom aka @Munificentbob

Example implementation of the Singleton Pattern in AS3

SingletonSkeleton.as

package
{

public class SingletonSkeleton
{
static private var _instance:SingletonSkeleton;

public function SingletonSkeleton( singletonLock:SingletonLock )
{
}

static public function get instance():SingletonSkeleton
{
if (!_instance)
_instance = new SingletonSkeleton(new SingletonLock());

return _instance;
}
}
}

class SingletonLock
{
}

Here, the idea behind the SingletonLock Class is to hide the key in the box, if you want to open it you have to use the getter.

As a use case, I choose to illustrate the Singleton Pattern with a SoundManager. (Sorry Bob…)

package engine
{
import flash.media.Sound;
import flash.media.SoundChannel;
import flash.media.SoundMixer;
import flash.media.SoundTransform;

public class SoundManager
{
private const MAX_TRACK:int = 16;
private var _tracks:Vector.<sound> = new Vector.<sound>(MAX_TRACK, true);</sound></sound>

// singleton instance
static private var _instance:SoundManager;

// audio channel
private var _channelBGM:SoundChannel = new SoundChannel;
private var _channelSFX:SoundChannel = new SoundChannel;

// tracks id
public static const BIP:int = 0;

public static const BGM1:int = 1;
public static const BGM2:int = 2;
public static const BGM3:int = 3;

public static const BONUS1:int = 4;
public static const BONUS2:int = 5;
public static const BONUS3:int = 6;
public static const BONUS4:int = 7;

public static const ACTION1:int = 8;
public static const ACTION2:int = 9;
public static const ACTION3:int = 10;
public static const ACTION4:int = 11;

public static const SPECIAL1:int = 12;
public static const SPECIAL2:int = 13;
public static const SPECIAL3:int = 14;
public static const SPECIAL4:int = 15;

//
private var _currentBgmIdx:int = -1;
private var _paused:Boolean = false;
private var _position:Number = .0;

public function SoundManager(singletonLock:SingletonLock)
{
}

static public function get instance():SoundManager
{
if (!_instance)
_instance = new SoundManager(new SingletonLock());
return _instance;
}

public function addRessource(sound:Sound, idx:uint = ACTION1):void
{
if (idx &lt; _tracks.length)
			{
				_tracks[idx] = sound;
			}
			else
			{
				throw new Error("ERROR: " + _tracks.length + " slots Max.");
			}
		}

		public function playSfx(idx:int = ACTION1):void
		{
			var s:Sound = _tracks[idx];
			if (s == null)
			{
				throw new Error("ERROR: No ressource for track " + idx);
			}
			else
			{
				_channelSFX = s.play(0, 0, _channelSFX.soundTransform);
			}
		}

		public function playBGM(idx:int = BGM1, repeat:int = 999):void
		{
			var s:Sound = _tracks[idx];
			if (s == null)
			{
				throw new Error("ERROR: No ressource for track " + idx);
			}
			else
			{
				if (idx != _currentBgmIdx)
				{
					_paused = false;
					_channelBGM.stop();
					_currentBgmIdx = idx;
					_channelBGM = s.play(0, 999, _channelBGM.soundTransform);
				}
			}
		}

		// PAUSE/RESUME BGM
		public function pauseBGM():void
		{
			if (!_paused &amp;&amp; _currentBgmIdx &gt; -1)
{
_position = _channelBGM.position;
_channelBGM.stop();
_paused = true;
}
}

public function resumeBGM():void
{
if (_paused &amp;&amp; _currentBgmIdx &gt; -1)
{
_channelBGM = _tracks[_currentBgmIdx].play(_position, 999, _channelBGM.soundTransform);
_paused = false;
}
}

public function set bgmVolume(value:Number):void
{
if (value &lt; 0)
			{
				value = 0;
			}
			_channelBGM.soundTransform = new SoundTransform(value);
		}

		public function get bgmVolume():Number
		{
			return _channelBGM.soundTransform.volume;
		}

		public function set sfxVolume(value:Number):void
		{
			if (value &lt; 0)
			{
				value = 0;
			}
			_channelSFX.soundTransform = new SoundTransform(value);
		}

		public function get sfxVolume():Number
		{
			return _channelSFX.soundTransform.volume;
		}

		// STOP (not pause) EVERY SOUNDS
		public function stopAllSounds():void
		{
			_paused = false;
			_currentBgmIdx = -1;
			_position = .0;
			SoundMixer.stopAll();
		}

	}

}

class SingletonLock
{
}

Usage

Step 1, embed your ressource

[Embed(source = "assets/Portal2-01-Science_is_Fun_part1.mp3")]
private const BgmScienceClass:Class;

Step 2, create your sound ressource

SoundManager.instance.addRessource( new BgmScienceClass() , SoundManager.BGM1 );

Step 3, just play when needed

SoundManager.instance.playBGM( SoundManager.BGM1 );

Demo

Credits
BGM 1 »Portal 2 OST
BGM 2 »LoneWolf DS
SFX » bfxr