the sound is repeating itself because timeline is repeating by default and you should add command stop()
in actionscript. You did not provide any source code so I'll assume you don't have any, in your case here is a sample program in actionscript that loops the background music.
import flash.events.MouseEvent;
stop();
var bgMusic:Sound = new BGMusic();
var bgChannel:SoundChannel = new SoundChannel();
bgChannel = bgMusic.play();
bgChannel.addEventListener(Event.SOUND_COMPLETE, loop);
startButton.addEventListener(MouseEvent.CLICK, startGame);
function startGame(e:MouseEvent):void {
gotoAndStop(1, "Scene 2");
}
function loop(e:Event):void {
bgChannel = bgMusic.play();
bgChannel.addEventListener(Event.SOUND_COMPLETE, loop);
}