The piezo buzzers or 'sounders' as refrenced above only generate sound at a single tone, their oscillator is built in. If you want a metronome tick, you basically just need to use a transistor on the output pin and generate the pulse to represent the tick. Start a timer that will create an interupt say roughly twice per second using the timer prescaller and the known system clock. Every time that timer overflows/triggers run a routine that turns the I/O line the transistor that drives the speaker is hooked up to on, and then immediatly turn it off, see what that gives you for output. Gradually increase the delay between on and off times untill you get the maximum perceived volume. Basically what you're trying to do is turn the I/O line off hard as soon as the speakers cone gets to it's maximum height. That will give you the highest possible impulse spike out of the speaker. You should end up with a nice satisfying tick.