Basically you need to do the following

Add a pause/play movie clip button to the fla just like the other buttons already there

Look at how the other buttons are added to the stage

In StageManager.as look at the sound code lines 149 - 155 (musicSnd.loadSound)

google something like "loadsound pause"

Now you need to take what you found googling and use the code to add a function to your pause/play button.