mixer

Often referred to as a "sound board", "mixing desk", or "console". A mixer takes multiple incoming sound signals (microphones, playback devices, other traducers, etc,), and allows the user to manipulate the incoming signal, the volume of each signal, manipulate the tone of the sound, and ultimately combines the incoming signals into a single sound signal (or very often a stereo output) coming out of the device into a PA system or monitoring devices (in most cases).