I'm a bit stumped too.
Knowing what this circuit is supposed to do would help; I don't suppose you know?
I do know that the ijnverters are all configured to behave as linear amplifiers but I don't know why the designer didn't just use op-amps like any normal person would.
N1, N2 and N3 form an amplifier with nearly open-loop gain, C8 provides a roll-off to prevent high frequency oscillation.
D1 and D2 are biased by the R3 and R4 to overcome the 0.7V forward voltage forming a prescion hlaf wave rectifier.
N4 acts as another amplifier with C3 acting as a low pass filter.
R6, R7 and C4 from an attenuator and low-pass filter, with T1 forming a peak detector.
N5 and N6 look like an oscillator.
Is BZ1 a piezo transducer rather than a buzzer with an internal drive circuit?
I think I've got it, it's a keys finder circuit!
When you whistle at it, it beeps back!
The piezo transducer acts as a microphone amplified by N1 - N3.
D1 and D2 (as I said before) form a precision rectifer.
When sound is detected the output of N4 is pulled low, discharging C4 and turning T1 on, which allows N5 and N6 to act as an oscillator which drives the piezo transducer.
The low pass filter action of C3 prevents the output from the oscillator from feeding back through the circuit and triggering it.