Most PC sound cards employ so-called single-supply designs running their chips only between Ground and a single positive supply, e.g. 5V or 3.3V. So typically

Most PC sound cards employ so-called single-supply designs running their chips only between Ground and a single positive supply, e.g. 5V or 3.3V. So typically for a headphone output the amplifier will swing around 1.5 or 2.5V or so. To get rid of this offset, a capacitor is used. This capacitor, together with the headphones impedance, will function as a RC hipass.