A new guide today in the Adafruit Learning System: Four Channel Audio Passive Stereo Mixer
In this project, you’ll learn how to build a passive mixer with four inputs and one main output to bring all of your audio devices together in perfect harmony. A passive mixer is an audio mixer that doesn’t require any power because it doesn’t have an amplifier. It’s a circuit of audio jacks with potentiometers that are wired up to let the audio input signals flow to an audio output.
This passive mixer build is a great introduction to audio projects and can be expanded upon for more customized builds.