Sound waves create pressure variations in the medium through which it travel. The microphone generates a voltage proportional to the pressure. You can consider the microphone as a pressure sensor, but working only for time varying pressures.
Objective
Digitize sound signals from a microphone, and measure its frequency. Use the Piezo buzzer or any other source of sound like a tuning fork.
Procedure
The microphone output is shown in figure.
Discussion
The ability to capture sound and measure it's frequency leads to several experiments. Making sound by blowing air in to one end closed tubes can be used for measuring the velocity of sound in air.