This video by RimstarOrg demonstrates how to build a Piezoelectric crystal speaker:
I make a loud speaker for a clock radio using a piezoelectric crystal. Actually I experiment with a variety of piezo crystals. That includes a piezoelectric igniter crystal from a BBQ lighter and a homemade/DIY rochelle salt piezoelectric crystal. I also try out a piezo buzzer from Radio Shack and also a crystal earpiece normally used with a crystal radio. To increase the voltage coming from the radio I first try stepping it up using a doorbell transformer and then I use a microwave oven transformer.
Via Hacked Gadgets.
You might be better off using the little transformer from a bug shocker racket that steps up from 3VDC to about 4kv’s of pulsating dc (I think). I get these from Harbor Freight Tools for about 3 bucks. A microwave trans is a bulky, heavy deal, not to mention ugly.