There is a fantastic book by a WWII veteran titled, "Crack and Thump" so named because the officer learned to judge, pretty accurately, how far away the Germans were based on the time between hearing the "crack" of a supersonic bullet going past and the "thump" ... the report from the actual bullet being fired.
If you hear a "crack" you are in deep trouble already, so hit the deck.
Here's a link to the book, by the way:
http://www.armchairgeneral.com/crack...ook-review.htm