how to save a sound object to local file for play later in adobe air ?

Question

in my application , I want to download a mp3 file for remote server ,then play it . now I'd like to save the sound to local , for play it from the file later , just like a cache . I've tried to extract the raw data to a file when the sound object load complete ,

sound.addEventListener(air.Event.COMPLETE, function(event){
 var cacheFileStream = new air.FileStream();
 var file = cacheDirectory.resolvePath("testfile1");
 cacheFileStream.openAsync(file, air.FileMode.WRITE);
 cacheFileStream.position = 0;
 var dataBuffer = new air.ByteArray();
 var targetSound = event.target;
 targetSound.extract(dataBuffer,targetSound.bytesLoaded,0);
 log(dataBuffer.length);
 cacheFileStream.writeBytes(dataBuffer, 0, dataBuffer.length);
 cacheFileStream.close();
        air.Introspector.Console.log('load complete');
    });

but how to play it?