Lowering pitch sound of a piezoelectric buzzer
A piezoelectric buzzer has a built-in oscillator that will make a fixed frequency sound when you apply power - you can pulse that sound by turning the power off and on, but you can't change the frequency.
There are also piezoelectric transducers, where you can drive the thing with an audio frequency and it will produce that frequency sound.  With these, you can change the frequency to some extent by changing the driving frequency - but i think most transducers will work best over a fairly narrow range of frequencies.