stopAllSounds() - assuming you are using Actionscript 2.
One quirk with this function. There needs to be 1 blank
keyframe in-between when you call it, and when you start a new
timeline sound. (Note, put a keyframe buffer at the beginning of
each scene, start the sound in the second frame - and call the
function just before you call to change the scene (from your button
press, or end frame))