From the ALSA wiki
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What is a Mixer?
A mixer is a device that controls the sound inputs and sound outputs on a soundcard. A mixer requires both hardware and software components. The hardware mixer is part of the electronics inside a soundcard. A software mixer is some software which provides a user interface for controlling the hardware mixer. Examples of software mixers include:
A software mixer does not control the hardware mixer directly; the ALSA sound driver is always an interface between the software mixer and the hardware mixer. This is because different soundcards have different interfaces to their hardware mixers. The purpose of the ALSA sound driver is to provide a standard interface for controlling the different hardware-mixer interfaces. This means one software mixer can work with many different soundcards. Typical examples of mixer controls are:
Master Volume (L,R) Bass Tone (L,R) Treble Tone (L,R) Line Input (L,R) PCM Volume (L,R) FM Synth Volume (L,R) CD/DVD Volume (L,R) Microphone Input Gain (L,R)
A software mixer usually has two separate controls for independently controlling the volumes of the left and right (L,R) channels of each stereo sound source. There may also be controls for mono sound sources, e.g. mono microphones. A mixer is implemented inside ALSA using a ctl-device.
For more technical information, see the page ALSAMixers.