How do I add music (mp3) to my flash movie?
please check the tutorials section.
it only shows how to add sounds =P from some library thing
The simplest way of doing things is to import sound files into your library. Then you can place them across into Timelines as per normal symbols. For example you can attach a click sound to the Hit state of a button, by making a keyframe and dragging the sound across into it.
I’ve found that adjusting the sound options when you export it from mp3 to ADPCM 44mH gives a much better quality, although you’ll have to watch out for your file size.
There are other ways of doing this of course. 
thats about the easiest way i’ve found kit
import the sound into a symbol and put that symbol on your stage, or attatch it to your button’s down state or whatever it is you want to do
hope this helps