Wow, took alout of time for some one to answer this one.
Make a multi manager that targets an ambiant generic thet plays that falling bomb sound and when it finishs that multi manager targets an Env_Explotion, it is very simple. Make more of these until you are stesfid. Now create a random targeting devise that starts on and targets those muli mangers.
Here is a link to an example map that explines how to make random triggering.
http://twhl.co.za/mapvault_map.php?id=2788