Adding and Manipulating Sound in AS3
For some reason I had a really tough time finding a quick and easy tutorial on how to add a .mp3 to to an AS3 project and lower the volume.
First, import your .mp3 to the library and set a linkage classname.

Right click on the .mp3 you have imported to your library and select 'Linkage'. Essentially, this allows you to address the .mp3 as a class.
You can then address it with the following code:
var snd:neonstart4 = new neonstart4();
var sc:SoundChannel = snd.play();
var st:SoundTransform = sc.soundTransform;
st.volume = .2; // set to 20% volume
sc.soundTransform = st;
var sc:SoundChannel = snd.play();
var st:SoundTransform = sc.soundTransform;
st.volume = .2; // set to 20% volume
sc.soundTransform = st;