I used a sound sensor based on the LM393 comparator IC. It is advertised as an Arduino sound sensor, sold for about 3-5 €.

Connect the + and G pins to 5 VCC and ground respectively: the L1 LED should turn on. It seems that the sensor works also at 3.3 VCC, beware thet changing the voltage requires a new threshold regulation.

The L2 LED will turn ON when the sound level is above the threshold, which is regulated with the trimmer potentiometer. The POT is a multi turn (may be 10?) potentiometer: turn the mini screw clockwise to be more sensible (to lower the threshold) until the L2 stays ON at normal room noise, then turn the screw counter-clockwise until the LED goes off. Clapping your hands you should see the L2 LED blink.

When the sound is above the threshold, a digital high signal is sent across the DO (digital output) pin.