Thecircuit in post #13 is not an oscillator since it has no feedback.

I'm under the impression that the circuit gets a pulsed signal from somewhere else. The more I look at it the more I think the 2K7 resistor is where the signal comes from. The circuit, which I knew nothing about before was drawn out, not knowing what the transistor was. Someone helped with that and in #13 I redrew the circuit to better represent the driver circuit for the beeper. Maybe. I'm still not sure. But as I said in another post I found a 5 volt regulator on the board, so probably what drives the beeper or chime or whatever it may be.

 

Certainly the circuit in post #13 is an amplifier/switch, to drive he piezo sounder. The five volt regulator probably powered the logic portion that drove the amplifier circuit.
It is likely that the logic circuit produced several different sounds.