How to connect microphone to an Arduino with an opamp?
I know it's old, but may still interest someone.
With a piezo contact mic, you may want to also connect two diodes allowing current to go from ground to mic output and from mic output to vcc. Under normal conditions there is not voltage over them, but a piezo can create some serious voltage spikes if hit, and this will protect your amp / arduino, so excess voltage can go to power supply rather than amp.