The Crumble can’t drive a piezo ‘musically’, but if you just need a buzz, there are a couple of ways to do it.
The first would involve writing a loop to toggle an output between HI and LO (without any delays). This should produce an audible tone, although it might be a little quiet.
The second way involves connecting the piezo to a motor output and setting the motor to 50% power. Because the motors are PWMed (Pulse Width Modulated), you should hear a 1kHz tone.
Hope this helps,