Whitenoise Signal from Microcontroller Amplification IC Advice
Recently faced the same problem driving a piezoelectric speaker.

Solution: Use a miniature audio output transformer... we selected the Hammond 104R, ~ US$20 per unit.



The cube in the middle is the Hammond 104 series (0.71 x 0.86 x 0.75 inches), rated for 500 mW.
If power output requirement is under 100 mW, the truly miniscule Hammond 101V might be a better option (0.45 x 0.45 x 0.48 inches).

Connection: 


High impedance side drives piezo
Low impedance side driven by op-amp as buffer / voltage follower
Buffer input driven from the microcontroller output.


Note that the transformer is used with the primary and secondary switched, compared to typical audio output transformer implementations where the low impedance side drives the speaker. These transformers are optimized for use in either direction.

Much cheaper alternative transformers are available, down to about $2 each, if weather resistance is not essential. These particular units are potted, and practically anything-proof, something we needed for our design.