Trying to analyze the audio attenuation.. Is this analysis correct?
It's just easier to find the transfer function Vout/Vin =

$$\dfrac{R_2 + \frac{1}{j\omega C}}{R_1+R_2 + \frac{1}{j\omega C}}$$  

$$=\dfrac{1+j\omega R_2C}{1+ j\omega C(R_1+R_2)}$$

At DC the T.F. reduces to 1 and at high frequencies it reduces to: -

$$\dfrac{R_2}{(R_1+R_2)}$$

If you want in-between values you need to get rid of the "j" term underneath by multiplying numerator and denominator by the denominator's complex conjugate....

.... Or do what every one esle does and use a simulator: -



R1 = 100 ohms, R2 = 1000 ohms and C1 = 1 uF