I think you might need to give us a little bit more info on how you are loading your sounds.

Are you placing the sounds on the timeline, or creating them dynamically with ActionScript 3.0?

If you're doing it dynamically, here's a good site to help you out. It goes through the basics of adding a SoundChannel (which is required for stopping particular sounds) and how to stop a sound: http://www.republicofcode.com/tutorials/flash/as3sound/

Section "4. Stopping a Sound" on that page is probably what you're looking for. The code that does it (taken from the link above) is below:

Code:
var mySound:Sound = new Sound();
var myChannel:SoundChannel = new SoundChannel();
mySound.load(new URLRequest("myFavSong.mp3"));
myChannel = mySound.play();

stop_btn.addEventListener(MouseEvent.CLICK, onClickStop);

function onClickStop(e:MouseEvent):void{
myChannel.stop();
}
Hope that helps.