How to choose a capacitor for filtering an audio signal?
For a DC blocking cap, the impedance of the cap should be small relative to other impedances over the intended frequency range.  For example, hi-fi audio goes down to 20 Hz.  For some headroom, it's a good idea to design your circuits with high pass filter rolloffs at 10 Hz or so.

The impedance magnitude of a capacitor is 1 / 2&pi;fC, with f in Hz, C in Farads, and the result in Ohms.  When this roughly equals whatever impedance is in series with the cap at 10 Hz, then it will pass 20 Hz and up well enough to be considered hi-fi.

For example, if 1 k&Omega; will be in series with the cap, then it needs to be about 16 &micro;F.