The piezo discs are pretty loud, you just have to drive them at the right frequency - about 2.5KHz usually.
EPE did a freezer alarm many years ago using a PIC 12C508 (before the F versions came out), which I built just as a matter of interest. I modified the design and software slightly by removing the ground connection of the piezo and connecting it to another I/O pin, and driving it bridged. And while it was fairly loud before, it was a LOT louder with the extra drive from bridging.
I sent the details off to EPE, and they printed the mods a couple of issues later.
Anyway, presumably you have a microwave oven?, does it beep?, can you hear it? - it almost certainly uses an identical piezo, and only fed single ended.