Need help understanding simple oscillator circuit
Basically this circuit transforms the variations of voltage v(t) of the photoresistor due to the light intensity in variations of current i(t) in the oscillator circuit. This transformation is done by the bipolar transistor, it is its function to amplify the v(t) signal.

The variations of i(t) change the frequency of the circuit oscillator circuit, it causes the different sounds. (example of a RLC circuit frecuency and current dependency )

The BJT must be configured correctly to work into the linear region (non-saturated), then if you change the photoresistor with a potentiometer, it  will do the same effect, but consider first the correct range of the potentiometer, other case maybe it won’t work, excess of base current could change the transitor state to the saturated region.