hi,

This must be related with the resonance induced by parallel inductors Oleg stated I guess.

If you disassemble a wristled-watch which has the piezo sounder inside, then you see a small inductor connected to it.
I did it once because was asked myself with the same question as you - "how does it produce so high sound volume with only 1.3V battery inside?"
And then I used this inductor for my application where a piezo sounder had been utilized (...because wristled-watch was broken with such research finally :)

Regards,
Oleg