there are probably several different ways to do this. here's one way, using a Timer
-you create a Timer which will run a function a specific amount of times, every specified time interval. for this , maybe have it run every second, for 5 times
your function just sets the volume to decrease by .2 - so running it 5 times fades it down to zero over 5 seconds
you will need to put an ENTER_FRAME listener to the stage, which will know when it enters every frame.
your function says: if the frame number( or frame label) is such and such, then cary out the code, which triggers the timer, which start our fade.
not too bad, right?
assuming you have your Sound and SoundChannel (depends if yo uuse an internal or external sound)set up correctly, you also need to establish a SoundTransform, which in this case will transform the volume.
here's some code to take care of all this from the point of declaring a Soundtransform object onwards.
you will see that the timer triggers every second (1000 = 1 second), 5 times,
and that it triggers on entering frame 20
var snTr:SoundTransform = new SoundTransform();