In professional audio, a mixing console, or audio mixer, also called a sound board, soundboard, mixing desk, or mixer is an electronic device for combining (also called "mixing"), routing, and changing the level, timbre and/or dynamics of audio signals. A mixer can mix analog or digital signals, depending on the type of mixer. The modified signals (voltages or digital samples) are summed to produce the combined output signals.
http://en.wikipedia.org/wiki/Public_address

PA je skraćenica za public address
A public address or "PA" system is an electronic amplification system with a mixer, amplifier and loudspeakers, used to reinforce a given sound, e.g., a person making a speech, a DJ playing prerecorded music, and distributing the sound throughout a venue or building.
http://en.wikipedia.org/wiki/Public_address