Two possible solutions:
1. This should stop all sounds currently being played. May not work for you if you need things like background music to keep playing after the transition. This is a workaround.
import flash.media.SoundMixer;
SoundMixer.stopAll()
2. Do it in MovieClip, which has the sounds you want to get rid of:
var myMovieClip:MovieClip;
var muteTransform:SoundTransform = new SoundTransform();
muteTransform.volume = 0;
myMovieClip.soundTransform = muteTransform;
Flash - ! , Adobe , , , .
user233232