Isn't C1 and C2 nulling the input signal?
Those capacitors are marked 101, or 100 pF, they have a reactance of ~80k Ohms 
 for a 20 kHz signal. 

They're there to shunt higher frequencies (radio and the like) to ground.

The reactance of a capacitor is calculated with the formula \$ Xc =  \dfrac{1}{ 2 \pi f C}\$