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It's 2 oscillators, end-to-end. Each oscillator is made by the NPN-PNP transistor pair connected with a feedback capacitor from the collector of the second to the base of the first. The first oscillator is a low frequency one, it provides the pulses which interrupts the tone from the second and blinks the LED. The second oscillator has a much smaller cap and provides the high-frequency tone to the speaker. The CDS cell pulls the base of that very first NPN transistor low so that when light shines on it, the NPN is "off". With that transistor off, the second transistor doesn't get any base current so it's off, too, and so are the third and fourth transistors - so the rest of the circuit is off and very little power is consumed.
The one on the collector of the second transistor? You need this resistor on the collector of that PNP to pull the voltage to ground when the transistor is off. Look up "Class A amplifier" to find out more.
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