When closing the switch, a working voltage of Ub = 4,5 V applies. The condenser can charge.
When reaching the threshold for the NPN transistor, the latter starts to switch through and to set free the basic current for the PNP transistor.
This one switches through as well and a sound of a certain frequence is being heard.
The frequence is the number of oscillations (switch on - switch off time or "oscillation periods") per second.
A high tune ( a high frequence) can be obtained when diminishing the resistor and condenser values. An increase of those values produces a lower-pitched tune.
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