Sound card
What we hear in speakers is an analog (current, voltage or electric charge) output of the signal. For example, stored on a hard disk digital code is transferred under a control of some application (for example Winamp) to the sound card. Up in there, a special chip, called digital-to-analog converter, changes the binary code into the analog sound. Afterwards, signal is sent to a jack (in modern audio cards it has the green color) where the speakers are connected. The result is a nice waveform of our favorite digital medium, e.g. an mp3.