Resonant frequency of piezo elements, and generating multiple tones
You can use the piezo to produce sound at a wide frequency range, but indeed it will be the loudest at the resonance frequency. If you look at a few more datasheets you'll find that some manufacturers do publish SPL versus frequency data, like Murata, for instance.




As you can see the spectrum can be very different between two piezo buzzers (the graphs are for types from the same PKM series), so it's worthwhile to compare many different types.

Further reading
Piezoelectric Sound Components, Murata catalog