The circuit uses four NAND gates of a 4011 package. In each oscillator, while one gate is configured as a straightforward inverter, the other has one input that can act as a control input. Oscillator action is inhibited if this input is held low. The first oscillator (ICla and IClb) has this input tied low via a high value resistor (Rl) that acts as a sensitivity control. Thus this oscillator will be disableduntil the control input is taken high. Any moisture bridging the sensor track will so enable the output which is a square wave at about 10 Hz
This in turn will gate on and off the 500 Hz oscillator formed by IClc and ICld. This latter oscillator drives the loudspeaker via R6, the Darlington pair formed by Q1 and Q2, and resistor R7.