If you want one that changes tone as the reflected IR changes you should sample the voltage across C4 and use it to tune an oscillator.
If you want the buzzer to operate only when a certain distance is reached, you should drive it from the output pin of U2B,C or D. You probably need a transistor driver to operate the buzzer, I doubt the IC can provide enough current on it's own.
Velleman have an IR barrier kit that uses the LM324 and has a buzzer, it has bc547s or similar on it, no idea what they do as I haven't seen the schematic. Funny, you want to add one, I wanted to take the buzzer off my kit and have it trigger a CD4026 instead.