Just completed a Velleman Voice Changer board, but it is not working. The Zenerdiode, LEDs and electrolytic capacitors are all installed correctly (so, not a polarity issue). Soldering joints are all good. The LEDs light up when the battery is connected. Changing the microphone sensitivity and/or the volume has no effect. The output voltage (at the speaker terminals) is a few millivolts (hence, no sound from the speaker). Any ideas? Bad microphone?