An audio mixer, also known as a mixing board or a soundboard, is used to control the levels of multiple inputs so you can balance the sounds correctly. Mixing is an important process when you record music or perform live so one instrument doesn’t overpower the others. Using a mixer may seem intimidating at first, but it’s not too hard once you know what the knobs do. After you connect your instruments or microphones, adjust the volume of each input until you find the mix you like!

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Source: https://www.wikihow.com/Use-a-Mixer