The timing for the siren comes from a 555 timer. A two transistor oscillator changes the frequency of the 555 timer and flashes the LED lights.
The two capacitors on the transistor side of the circuit will alternate between being charged and discharged and thereby turning the transistors ON and OFF. When a transistor is ON it allows current to flow through it so that the LED above the transistor will light up.
The feedback to the CTRL of the 555 timer comes from the right hand side of the light oscillator circuit.
The speaker is simulated with the 8 ohm resistor and 400 uH coil.
