The gate at the bottom of the schematic diagram acts as a linear low frequency amplifier, due to the negative feedback resistor R3. But I think IC1d is only an oscillator, so how IC1d acts as a low frequency amplifier?

 

It appears that what you have is two oscillators each oscillating at a slightly different frequency. The resulting output is the difference and the sum of the two frequencies. The lower frequency is the one that is dominant.

hgmjr

 

The 1Meg feedback resistor forces the output voltage to equal the input threshold voltage. Since the 4011 has voltage gain, it will act as an amplifier.

 

IC2a and IC2b is the first slow oscillator that gates the second audio oscillator IC2c and IC2d on and off.

 

I should add that Ic1d is not an oscillator. (Unless I use it. Then all amplifiers are oscillators.)

 

I seems to me that IC1d is furnishing feedback to re-trigger the oscillator in order to control the overall period of the oscillation.

hgmjr