基本的演算法是：【位元速率】（kbps)=【檔案大小】（位元組）X8/【時間】（秒）/1024

package
{
	import flash.display.Sprite;
	import flash.events.Event;
	import flash.events.MouseEvent;
	import flash.media.Sound;
	import flash.net.FileFilter;
	import flash.net.FileReference;
	
	public class GetMp3BitRateDemo extends Sprite
	{
		private var _fileRefer:FileReference;
		
		public function GetMp3BitRateDemo()
		{
			stage.addEventListener(MouseEvent.CLICK,onClick);
		}
		
		private function onClick(e:MouseEvent):void
		{
			_fileRefer = new FileReference();
			_fileRefer.addEventListener(Event.SELECT,onSelect);
			_fileRefer.addEventListener(Event.COMPLETE,onComplete);
			_fileRefer.browse([new FileFilter("Images (*.mp3)", "*.mp3")]);
		}
		
		private function onSelect(e:Event):void
		{
			_fileRefer.load();//本地載入
		}
		
		private function onComplete(e:Event):void
		{
			if(!_fileRefer || !_fileRefer.data)
				return;
			
			var mp3:Sound =  new Sound();
			mp3.loadCompressedDataFromByteArray(_fileRefer.data,_fileRefer.data.length);
			var bitRate:Number =  Math.round((_fileRefer.size * 8) / (mp3.length/1000) / 1024);
			trace("bitrate == " + bitRate);
			_fileRefer.cancel();
			_fileRefer = null;
		}
	}
}
 

Enjoy it.