I would suggest the timeline approach for sound interaction for cases where you may need backward compatibility for Flash 4.

create a simple mc with:
  • stop action in frame 1
  • sound in frame 2 set to event sync
  • sound set to loop 999 times
  • stop action in frame 3
  • same sound in frame 4 set to stop sync


use tellTarget("target path to mc containing sound") and gotoAndPlay() to both stop and start sounds.

ex.
Code:
//on button
on(rollOver){
    tellTarget("/soundmc"){
         gotoAndPlay(2); // start sound
    }
}

//on button
on(rollOver){
    tellTarget("/soundmc"){
         gotoAndPlay(4); // stop sound
    }
}
Read the Timeline Approach tutorial at sonify.org for more details

http://sonify.org/cgi-bin/tutoriallinks/jump.cgi?ID=113