For Astronaut & Isidore (also see this post) I wanted to control not only the volume of a sound, but also the pitch. Since this is not supported (yet) in Actionscript, I searched and found this great class made by Andre Michelle: MP3Pitch.as. But because I wanted to use short sound effects I also wanted the sound to loop, which was not supported. So I changed it a little (now you can also start and stop the sound more easily). You can download the version with my additions here.
Be careful not to start too many sounds at the same time (three sounds already slowed down my computer), since every sound has its own SampleData stream. (If you want to do this I think it is best to rewrite the class and mix the different sounds into one SampleData stream.)
Another interesting software library to manipulate sound with is Antti Kupila’s SoundFX library, which makes it possible to add different kinds of filters to an MP3.