using sound to begin loop?

I am attempting to start a loop with the prompt of a gunshot.
im hoping that i am able to use the spike in volume to begin the code, and i was wondering if that is possible and what sort of coding i would use.
i was also hoping to reverse a speaker to use as a mic, if that is also possible with arduino.
any suggestions would be lovely.

deja vu

http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1237867807