An audio mixer is device which is used to mix several sounds together. The user can control the volume on each channel or sound source to be as loud as they want. The amount of channels may vary depending on the mixer. Different sources may be used such as microphones, sound cards and CD players. Usually mixers have pre-amplifiers for microphones and inputs may be balanced. It is a device that allows you to edit the pitch and other aspects of sound.
Each channel has their own volume control but also there is main volume control that changes all channels the same way. Some mixers will also have an equaliser.