R3 and R4 create a voltage generator by the equation:
VB = VCC*R4/(R3 + R4), for the values in the circuit: V = .9V.
The output resistance of the voltage generator is ~= 240K, the smaller of the two resistors.
IB would then be the generator voltage minus .6V, which is the approximate VBE of the transistor, divided by the generator resistance: IB = (.9 - .6)/240K = 1.25uA.
Assuming Beta of 100; IC = 125uA
And finally, the collector voltage in the quiescent state: VC = 9 - 125uA*3.3K = 8.6V.
So the transistor is just barely conducting. A signal from the mic causes the transistor to conduct a higher current, which is then sensed by the first timer, and causes it to trigger.
The -3db frequency of the C1 network is, using time constants is:
1/2pi*R, where R is R2 + ((R1 + R3)||R4)
And for C2:
1/2pi*R, where R is R5+R6.