This circuit produces the sound of a beeper like the one in pagers which produces a "beep-beep" sound. Basically the circuit consists of a 555 timer oscillator which is turned ON and OFF periodically. The first IC(left) oscillates at about 1Hz. The second IC is turned ON and OFF by the first IC. The first IC determines how fast the second IC is turned ON/OFF and second IC determines the tone of the final output. By varying the VR1, the changeover rate can be adjusted. By varying VR2 the tone can be adjusted.
If you know something about electronics, you can try replacing the 2nd 555 IC circuit with a piezoelectric buzzer. This saves one IC and associated components but the buzzer cannot give a loud sound as the speaker and also its tone cannot be varied.