Changing the pitch of a looped sound at runtime with Actionscript.

package sound
{
	import flash.events.Event;
	import flash.events.SampleDataEvent;
	import flash.media.Sound;
	import flash.media.SoundChannel;
	import flash.media.SoundTransform;
	import flash.net.URLRequest;
	import flash.utils.ByteArray;
 
	/**
	 * @author Andre Michelle (andr...@gmail.com)
         * Modified by Zach Foley aka The Plastic Sturgeon
	 */
	public class MP3Pitch
	{
		private const BLOCK_SIZE: int = 3072;
 
		private var _mp3: Sound;
		private var _sound: Sound;
 
		private var _target: ByteArray;
 
		private var _position: Number;
		private var _rate: Number;
		private var repeat:SoundChannel;
		private var _volume:Number = 1;
		private var byteArray:ByteArray;
 
                // Pass in your looped Sound
		public function MP3Pitch( pitchedSound: Sound)
		{
			_target = new ByteArray();
			_mp3 =  pitchedSound;
 
			_position = 0.0;
			_rate = 0.0;
 
			_sound = new Sound();
			_sound.addEventListener( SampleDataEvent.SAMPLE_DATA, sampleData );
			repeat = _sound.play();
		}
 
		public function get rate(): Number
		{
			return _rate;
		}
 
                // Also added a handy volume setter
		public function set volume( value: Number ): void
		{
			_volume = value;
			repeat.soundTransform = new SoundTransform(_volume);
		}
 
                // use this to set the pitch of your sound
		public function set rate( value: Number ): void
		{
			if( value < 0.0 )
				value = 0;
 
			_rate =  value;
		}
 
		private function sampleData( event: SampleDataEvent ): void
		{
			//-- REUSE INSTEAD OF RECREATION
			_target.position = 0;
 
			//-- SHORTCUT
			var data: ByteArray = event.data;
 
			var scaledBlockSize: Number = BLOCK_SIZE * _rate;
			var positionInt: int = _position;
			var alpha: Number = _position - positionInt;
 
			var positionTargetNum: Number = alpha;
			var positionTargetInt: int = -1;
 
			//-- COMPUTE NUMBER OF SAMPLES NEED TO PROCESS BLOCK (+2 FOR INTERPOLATION)
			var need: int = Math.ceil( scaledBlockSize ) + 2;
 
			//-- EXTRACT SAMPLES
			var read: int = _mp3.extract( _target, need, positionInt );
 
			var n: int = read == need ? BLOCK_SIZE : read / _rate;
 
			var l0: Number;
			var r0: Number;
			var l1: Number;
			var r1: Number;
 
			for( var i: int = 0 ; i < n ; ++i )
			{
				//-- AVOID READING EQUAL SAMPLES, IF RATE < 1.0
				if( int( positionTargetNum ) != positionTargetInt )
				{
					positionTargetInt = positionTargetNum;
 
					//-- SET TARGET READ POSITION
					_target.position = positionTargetInt << 3; 	 					//-- READ TWO STEREO SAMPLES FOR LINEAR INTERPOLATION 					l0 = _target.readFloat(); 					r0 = _target.readFloat(); 					l1 = _target.readFloat(); 					r1 = _target.readFloat(); 				} 				 				//-- WRITE INTERPOLATED AMPLITUDES INTO STREAM 				data.writeFloat( l0 + alpha * ( l1 - l0 ) ); 				data.writeFloat( r0 + alpha * ( r1 - r0 ) ); 				 				//-- INCREASE TARGET POSITION 				positionTargetNum += _rate; 				 				//-- INCREASE FRACTION AND CLAMP BETWEEN 0 AND 1 				alpha += _rate; 				while( alpha >= 1.0 ) --alpha;
			}
 
			//-- FILL REST OF STREAM WITH ZEROs
			if( i < BLOCK_SIZE )
			{
				while( i < BLOCK_SIZE ) 				{ 					data.writeFloat( 0.0 ); 					data.writeFloat( 0.0 ); 					 					++i; 				} 			} 			//-- INCREASE SOUND POSITION 			_position += scaledBlockSize;                        // My little addition here: 			if (_position > _mp3.length * 44.1) {
				_position = 0;
				_target.position = 0;
			}
		}
	}
}

pitched = new MP3Pitch(new CarLoopLow())

pitched.rate = 0;

pitched.rate = 0;