OK, I drew up the circuit as MarkT described it in LTSpice.
Does that output look reasonable (green line is voltage from the pin, blue line is voltage across the speaker, which is the 8 Ohm resistor)? The pin outputs a 450Hz square wave.
Is it possible to get something resembling a pure sinuosoid at the output without excessive complexity? Will that signal produce a decent tone through the speaker?
Also, I'm not entirely sure I'm operating anywhere near the max power output of the speaker. Power = V^2/R, but for AC, do I take V as V_max - V_min (which is about 1.4V), or is it just the peak voltage (about 0.7V).
If the former, the power output is about 0.25 Watts, which is what I'm after. If the latter, it's only 0.07 Watts, which is far too low. If it's Vrms of the peak, then it is 0.125 Watts, which is still pretty low.