Digital sound reproduction is usually achieved by multi-channel DACs, able to play multiple digital samples at different pitches and volumes, optionally applying real-time effects like filtering or distortion. Multi-channel digital sound playback can also be used for music synthesis if used with a digitized instrument bank of some sort, typically a small amount of ROM or Flash memory containing samples corresponding to the standard MIDI instruments. (A contrasting way to synthesize sound on a PC uses "audio codecs", which rely heavily on software for music synthesis, MIDI compliance and even multiple-channel emulation. This approach has become common as manufacturers seek to simplify the design and the cost of the sound card itself).

Most sound cards have a line in connector where the sound signal from a cassette tape recorder or similar sound source can be input. The sound card can digitize this signal and store it (controlled by the corresponding computer software) on the computer's hard disk for editing or further reproduction. Another typical external connector is the microphone connector, for connecting to a microphone or other input device that generates a relatively lower voltage than the line in connector. Input through a microphone jack is typically used by speech recognition software or Voice over IP applications.

USB sound cards

While not literally sound cards (since they don't plug into slots inside of a computer, and usually are not card-shaped (rectangular), there are devices called USB sound cards. These attach to a computer via USB cables. The USB specification defines a standard interface, the USB audio device class, allowing a single driver to work with the various USB sound devices on the market.

Other outboard sound devices

USB Sound Cards are far from the first external devices allowing a computer to record or synthesize sound. For example, devices such as the Covox Speech Thing were attached to the parallel port of an IBM PC and fed 6- or 8-bit PCM sample data to produce audio. Also, many forms of professional soundcards (audio interfaces) have the form of an external Firewire or USB unit, usually for convenience and improved fidelity.

Also, soundcards utilizing the PCMCIA cardbus factor are popular in the early days of portable computing where laptops and notebooks do not have onboard sound. Even today, while rare, these cardbus audio solutions are still used in certain setups where the onboard sound solution of the notebook or laptop is not up to par with the owners' expectations or requirements, and are particularly targeted at mobile DJs, with units providing separated outputs allowing both playback and monitoring from one system.