Emitter follower/Common collector confusion
C1 is used to superimpose your signal on top of the base bias voltage. C2 acts as a dc blocking component which blocks the DC component of the emitter voltage and lets through the buffered signal. C1 in connection with R1||R2||(Input impedance of the transistor) forms a high pass filter.

Neglecting C2 for now here's how you should pick up a value for capacitor C1. The input impedance of an emitter follower is BetaRE, where Beta is the current gain of the transistor, and RE the resistance measured across the emitter and ground. As I said, C1 together with R1||R2||BetaRE forms a high pass filter. If Beta*RE is large compared to R1||R2 then you can write simply

$$H(w)=\frac{R_1||R_2}{R_1||R_2+1/jwC_1}$$

where its absolute can be written as

$$|H(w)|=\frac{R1||R2 \times wC_1}{\sqrt(1+R_1||R_2 \times wC_1)}$$

Now if you want to pass all signals of interest then you should choose C1 so that

$$C1 &gt;\frac{1}{2\pi f R_1||R_2},$$

where f is the frequency of the signal of interest. For example, for R1=10k, R2=110k, RE=100k, and f=400Hz, you'll need to set C1 equal to 0.1uF or so.