First of all, you need the sound to export for actionscript:
-Right Click your sound in the library
-Select 'Linkage'
-Check 'export for actionscript'
-Type something in the Identifier box eg 'mySound'
Now the button script:
on(press){
mySound = new Sound(this);
mySound .attachSound("mySound");
mySound.start(0, 1);
}
That will play the sound once when the button is pressed. There's lots of things you can do with this, one useful one is to change the number 1 in the brackets to get the sound to play more than once eg mySound.start(0, 5); will play it 5 times.
Hope this helps :)