When mains power os applied to the circuit, the 4K7 resistor resists the voltage and drops it to lower levels suitable for operating the DC electronic circuit.
The diode rectified the low level AC to DC while the capacitor filters and smoothens the rectified DC.
Initially the voltage reaches the base of the transistor which instantly conducts and pulls the one half winding of the transformer to ground, forcing a strong induced current across the secondary winding.
However the moment this happens, the entire voltage is shorted to ground via the transformer winding and this eliminates any biasing voltage at the the base of the transistor and it fails to support the conduction.
The transistor releases the transformer activation which reverts a strong back emf to the secondary winding.
However the moment the transistor stops conducting, the voltage at it’s base is restored and the cycle repeats again.
This repeated pulsation of the transformer induces a strong back emf oscillation at the secondary of the transformer, which is amplified over the connected loudspeaker.
The associated components, the 10 resistor and the 0.1 capacitor performs the feedback function for keeping the transistor active with a certain foxed frequency range.
The frequency of the circuit may be adjusted through the 4k7 pot and the 0.1uF capacitor so that any desired tone can be achieved at the output over the speaker.