Use an Arduino to monitor the amplified signal from an electret microphone. When a threshold is exceeded, plays a ditty on the piezo speaker.
Here's a quick video of the circuit in action:
The electret microphone is biased to about 1V, and the AC component of the signal tapped via C1.
The microphone signal is then fed to the LM324 setup as a subtracting amplifier:
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positive input is set to Vcc/2 = 2.5V
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microphone signal fed to negative input
Vo = G1 * Vmic + G2 * Vcc G1 = -R2/R1 = -330kΩ/2.2kΩ = -150 G2 = (R1 + R2)/R1 * R4/(R3 + R4) = (2.2kΩ + 330kΩ)/2.2kΩ * 22kΩ/(22kΩ + 22kΩ) = 75.5
When no microphone signal, we are reading Vcc/2 at the output.
But it is really sensitive to changes in the microphone signal. Basically every 1mV results in a 150mV change in the output.
The amplified signal is read by the Arduino, and when the signal exceeds a threshold, the Arduino triggers a tune played on the piezo speaker.
Other amplifier configurations are possible. For example the ElectretADC demo uses a non-inverting amplifier configuration.
NB: for simplicity, the circuit is powered from the Arduino 5V pin. That works fine, although it is equally possible to use a separate power supply (but ensuring there is a common ground connection).