How do I assign a keyboard event to a button?

Just use a

stage.addEventListener(KeyboardEvent.KEY_DOWN, keyPressed);

function keyPressed(event:KeyboardEvent):void {

     //trace(event.keyCode);

     if(event.keyCode == ???){

          // play the sound

     }

}

Where I show the "???" you would replace with the keyCode for the key you want pressed for that sound.  To determine the keyCode you can use the trace line that I have commented out.

Note that when testing in Flash you often need to disable the keyboard shortcuts in the Flash Player in order to be able to use alot of the keys.  You will find this option in the Control menu options in the player.