ActionScript 3 Invalid Sound Problem

import flash.display.*;
import flash.events.*;
import flash.text.*;
import flash.utils.getTimer;
import flash.utils.Timer;
import flash.media.Sound;
import flash.media.SoundChannel;
import flash.ui.Mouse;


/*
the aim of this program is to have a timer that infinitely repeats, and at every 50
seconds, it plays a sound. Unfortunately, I cannot get the countdown to work nor
the sound to play after the 50 seconds. The Output will trace "Timer Started", but will trace this error every 50 seconds:

ArgumentError: Error #2068: Invalid sound.
    at flash.media::Sound/play()
    at Untitled_fla::MainTimeline/playSound()
    at flash.utils::Timer/_timerDispatch()
    at flash.utils::Timer/tick()

Thanks for your help
*/

//variables
var timeLeft:uint = 50000;
var timer:Timer = new Timer(timeLeft,0); //fires infinite times
var beepSound:Sound = new Sound();
//event listeners
startButton.addEventListener(MouseEvent.CLICK, startTimer);


function startTimer (e:MouseEvent)//start timer, start countdown, disable button
{
    timer.start();
    trace("Timer Started");
    startButton.visible = false;
    timer.addEventListener(TimerEvent.TIMER, playSound);

}

function playSound (soundObject: Object)
{
    beepSound.play();
    trace("Sound Played");
}