What is Sound card mixer?

A sound card mixer is the analog part of a sound card that routes and mixes sound signals. This circuit receives inputs from both external connectors and the sound card's digital-to-analog converters. It selects or mutes, amplifies these signals, adds them together, and finally routes the result to both external output connectors and the sound card's analog-to-digital converters. Different mixing schemes are in use, but the ones implemented in most IBM-PC compatible computers today are variants of a scheme defined in Intel's AC'97 Audio Component Specification.