Double check.
The Piezos might not register on a Multimeter. Get a flashlight battery [ 1.5 volts ] and a couple of wires. Connect the earth or positive to one terminal of the speaker and connected the other terminal to the other polarity of the battery intermittently by tapping the connection on and off. If the speakers any good you'll get a clicking sound/response. [ there's no danger of damaging the speaker from doing this ]