Electret Microphone Preamp Circuit
Negative Feedback
It gives you:

better linearity
lower gain
even lower output impedance
opportunity for frequency-dependent gain

As shown, you get DC feedback via R1+R2.  But R1/C1 forms a low pass filter for signals coming from the collector and going to the base.  Less negative feedback means less reduction in gain for high frerquencies.  Cancel out all those double-negatives and you're left with a high-pass filter.
The transistor below corresponds to Q5 in your drawing.



simulate this circuit &ndash; Schematic created using CircuitLab