There's an oscillator that creates AC current from the DC supplied by the batteries. The AC is needed to step the voltage up to the 300 or so volts that the flash tube needs, and that voltage is rectified back to DC and used to charge the capacitor (which can deliver a lot of DC current in a very short time). The sound you hear is mechanical vibrations caused in the transformer as the voltage is stepped up -- the transformer converts the low-voltage AC from the oscillator to changing magnetic fields, which, in turn, induce higher-voltage AC current in a second coil of wires on the transformer. The changing magnetic fields distort the transformer very slightly, and those mechanical distortions are what you are hearing. As for the changing frequency, that has to do with the amount of voltage (think of it as "electrical pressure") the oscillator is working against, which changes the operating conditions of the oscillator circuit.