Ok, I would recommend you look for a sound sensitive module or a sound activated module, these things retail so cheap its not worth doing it yourself, plus most have sensitivity control and a realtivly good enclosure for water resistance, bumps etc...
Hope that helps...
They consist of a mic, some resistors, capacitor, diode and transistor at basics and can be expanded to include ics, relays and trimpots for more technical/specific control...
