(Electret Microphone) Why is input taken from the capacitor?
The capacitor is there to block the DC voltage from feeding into the A/D input of the microcontroller.

Inside the electret microphone there will be a FET that can conduct current to GND as sound impinges in the capacitive diaphram of the mic. The external resistor provides the bias current for the FET. Variation of the current flow in the FET is also seen as a variation of current in the resistor. This translates to a voltage variation across the resistor.