Adding two sound waves having nearby frequencies generate beats. The beat frequency is the difference of the two frequencies. We are using two Piezo buzzers, one driven by a 3300 Hz sine wave and the other by a 3400 Hz square wave (it would be better to use two sine waves, if available). The experiment setup is shown below. The microphone is mechanically supported by a piece of wire fixed on the I2C socket.
The distances between the microphone and the buzzers are adjusted to get good beat envelopes. A Fourier transform shows both the inputs.