You might also find the appnote from Measurement Specialties interesting/useful that discusses interfacing to piezo sensors.  Pertaining to this, search for the first instance of "protect" in the PDF.  You might also find a bazillion other uses for your piezo speaker that you hadn't anticipated...

http://www.meas-spec.com/myMeas/download/pdf/english/piezo/Interfacing%20to%20Electronics.pdf

regards,
-e

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