When sound pressure moves the diapharam in, the capacitance increases, and when it moves out, the capacitance decreases.
The change in capacitance results in change in out put voltage of microphone. Equation 1 shows that if C increases, V will decrease and if C decrease, V will increase
Where V=Voltage across the capacitor in volts.
Q= charge in coulombs
C= capacitance in farad