ActionScript 3.0 :: Play Movie Clips Embedded Inside A Movie Clip On The Main Timeline?

I have my flash document set up in the following way: On the main timeline I have my buttons that I want to play movie clips embedded inside a movie clip on the main timeline.However, when I click each different button, the current movie clip stops playing and then the movie clip corresponding to the button plays.

I want them all to be able to play at once. So for example you push button 1 and the alien plays, but half way through you could click button 2 and the alien clip would keep playing, but also the helicopter clip would begin to play as well.Here is the code I have so far:

building_btn1.addEventListener(MouseEvent.CLICK, alien);
building_btn2.addEventListener(MouseEvent.CLICK, helicopter);
function helicopter(e:MouseEvent):void {[code]......