Sound function?

I have a 4 buttons in my movie that I have added sound to. The sound has been added to play on roll over through action script. _root.thissound.start(0,1);.
My question is how do i prevent the sound from starting again on rollover, when it is already playing? I want the sound to finish completly before starting over again. Do I have to write a function? Could some one point me in the right direction as I have never done anything like this before. Thanks JenWhitie:egg: