Microphone amplifier with opamp doubts
The input impedance of the op-amp is zero, courtesy of R2 and C2, and its huge open loop gain. It's called a virtual ground input.

The output current of the microphone sees two impedances in parallel, R1 to AC ground (VCC), and C3 to AC ground (op-amp virtual ground). Its output current will split between them according to the ratio of their impedances. At very low frequencies, all the signal will go into R1, as C3 is very high impedance. At higher freuqencies, as C3 becomes lower impedance, all the current goes through C3.