optimize a download

Hello. I have this code where I download 54 (short) sound files. I also have a button that, while downloading, you can go back (in case the user has made the wrong file)

when I start downloading a group of files and hit the back button and then re-download the same group of files, the load is considerably faster!! (I do not know why!)

Is there a way to optimize this download so that it goes fast also the first time I hit it?

Thank you very much!

function load_soundsF():void{
	i = current_index;
	this[soundZ[i]] = new Sound();
	this[soundZ[i]].load (new URLRequest(this[Data.so.data.sound+i]));
	pantallaCarga.progressNumber.text=i + " / 54";
	pantallaCarga.carga_text.text = "Cargando "+ Data.so.data["sound"].split("_").join(" ")+"....";

	this[soundZ[i]].addEventListener(IOErrorEvent.IO_ERROR, onError);
	this[soundZ[i]].addEventListener(Event.COMPLETE, imagenCargada);
	
	function onError(event:IOErrorEvent):void {
		trace("ioErrorHandler");
		dispatchEvent(errorA);
	}
	function imagenCargada(event:Event):void{

	// load progress has advanced another 1/54th. update your loadbar.
		current_index++;

		if(current_index<54){
			load_soundsF();
		}
		else {
			//pantallaCarga.formato_1.text = Data.so.data["sound"].split("_").join(" ")+" cargado con exito";
			pantallaCarga.gotoAndStop(1);
		}

	}
}

pantallaCarga.back_button.addEventListener(MouseEvent.MOUSE_DOWN,back_home);

private function back_home(e:MouseEvent):void{
	dispatchEvent(backE);
}