You'll want an electret mike to get the sound input.
You'll need to amplify the output of that electret, which would require at the very least a transistor (I'd recommend a 2N4401, very reliable) but preferably an op amp, such as the LM386.
You'll need a servo, of course, and the requisite circuitry to drive a servo (connecting it straight to the arduino is a good way to get a fried arduino). I recommend the adafruit motor shield; it has all the drivers on it to safely use servos, steppers and standard DC motors with an arduino: http://www.adafruit.com/products/81
Then of course you'll need a button--a cheap momentary SPST will work.
And of course you'll need all the passive components needed to make all this work, like biasing resistors for your transistor or op amp, capacitors for power stability and smaller capacitors for your amplifier, a pullup resistor for your button, etc.