There is a simple way to adjust all global sound in Actionscript 3. This encompasses all playing loops, sound effects and such.
All you need is the following code:
// volume limits are 0 to 1 (0.1,0.2,0.3,...,1)
SoundMixer.soundTransform = new SoundTransform(0);
Of course change the number sent to the SoundTransform constructor to anything between 0 and 1 (0.5 would be half max volume, 0 is muted.)