You need to make the sound file in your library available for actionscript.
Give it a Class name ex. MySound
...
var mysound:MySound = new MySound();
var soundPLaying:Boolean = false;
function checkingProgress(event:ProgressEvent):void{
var procentLoaded:Number = event.bytesLoaded/event.bytesTotal*100;
ptxt.text= procentLoaded +"%";
if(procentLoaded > 25 && !soundPlaying){ //check if loaded mor then 25% and sound isn't playing
mysound.play();
soundPlaying = true;
}
if(procentLoaded == 100){
this.gotoAndPlay(1,"Video-Int");
}
}
...
if you need to do some extra stuff like pause, change volume etc you will need a SoundChannel Object