Using Hittest And Attaching Sounds?

I am working on a game that involves a play head (called playHead) moving along the screen and when it encounters various movie clips it triggers a sound to be played. It is all going well except that the sound seems to want to play more than once, so there is a staggered wierd sound. Is is possible to make the sound only play once and also it needs to play until the the sound has finished. (the sounds are all very short).Here is the code:
 
onClipEvent(enterFrame){    if (this.hitTest(_root.playHead)){            _root.sound15.stop();    _root.sound15.attachSound("over");    _root.sound15.start();            }       }