Thanks for your help, Zero999
I choose to not use a micropower op-amp because I would need to wait for ordered parts and the op-amp would likely need a PCB instead of through-hole which I can make do with immediately. Gramma can't wait!
The simulated doorbell tone signal to be picked up by the mic is, <60mV P-P at the mic output but varies, of course, with the distance of the mic to the intercom box, I haven't considered that a 12-bit ADC could use the 1.024V FVR and have enough resolution to easily identify the bell sound's signature waveform points.
The freq. of operation is underestimated. and the BW is better described as the audio spectrum, and so would have to have a much wider bandwidth than 2KHz to display the waveform without distortion: The belltone heard by the mic is a 700-mS period burst of sound that my scope shows is made up of a 2-mSec period waveform that repeats every 2-mSec and the bell waveform has distinctive and consistent peak amplitudes every 2-mSec.
Please see the attached amplified mic output scope snapshot of the bell ring. Unamplified, my scope cannot display clearly the waveform to work with. My 200-MHz scope is basically useless to see clearly<50mV/div signals because of unavoidable HF noise below 100mV/div swamping out the signal of interest.
Obsession: BatV/2 gives the collector quiescent point the largest swing in voltage output to the A2D without clipping. The ADC is using BatV as the reference.
The mic only uses 40uA of current, and while the MCU is in sleep mode the A2D uses only about 60uA continuous running, but my choice of ~25uA is based the goal of using the least amount of battery current, but, you are correct in assuming my choice is somewhat arbitrary, but was chosen to be comparable to the current drain of a micropower op-amp.