Simplest Approach: Trigger Sound File When Loud Sound Detected

Envision this: A loud sound is detected and as a result another sound is played.

My first inclination was to use a Piezo to detect when a loud sound was detected then trigger a transistor controlling power to a mini-amp.

I want the project to be as compact as possible, so using both Processing and Arduino is not what I am hoping to do. Consequently having an iPod hooked up to a miniamp playing a loop seems best. Ideally the entire project (mini-amp, speaker, arduino, battery, circuit) will all fit in a 6"x6"x2" enclosure.

Has anyone done a similar project by chance? I know using a Raspberry Pi running Processing would be better, but it adds to the bulk of the project and it needs to be as compact as possible.

Suggestions? Comments?

Get a wave shield from Ada Fruit.

What an idiot I am. I totally forgot about the wave shield.

Thanks very much. Pretty much finishes the project for me!