The output voltage of the mike is quite small and that mike can pick up noise easily.  To reduce this problem, a balanced mike input can be added.  It compares the two input signal ( +ve and gnd ) and cancels away any interference signal common to the two inputs and amplify the mike input signal.  This instrumentation amplifier has an extremely high CMRR (common mode rejection ratio) with a very high input impedance and finite stable gain.  Again the input capacitance is to block off any DC component in the signal and the potentiometer is to control the input volume so as to allow usage with any mike.