Single microcontroller pin high-voltage piezo speaker driver, what are the pitfalls?
Looks as if it's not intended as an amplifier, but a maker of loud noises (square waves only) such as for alarm signals. 

I suppose you could drive it from a PWM output and it would sort-of work; the only place you could put any filtering would be in series with the speaker itself.

Because the other three transistors are driven by Q2, they cannot switch until after Q2 has switched, and that asymmetrical delay may do bad things at PWM switching frequencies. A little thought will show that Q1 cannot turn off until AFTER Q2 has turned on, for example; creating a momentary short circuit across the PSU...

So my verdict would be: it's fine for alarms, but needs more analysis or testing before serious use in PWM applications.