Audio and Modular Signal Levels - BleepLabs/Arduino-Light-And-Sound GitHub Wiki

The Teensy audio adapter has a line and mic input. Only one can be used at a time.

The line input is stereo and can be attached to a jack to accept most audio signals. This includes guitars but you might have to amplify the signal with a booster pedal or a simple mixer before sending it into the Teensy. Going over the 3.3 limit of the device will result in clipping but you won't damage the device UNLESS you send in a modular level signal.

Modular audio is -5V to 5V which is above the safe level to send into it. You'll need to attenuate and buffer the signal first with a circuit like this.
To deal with simple gates and triggers these circuits can be used. Note that the signals will be flipped but this can easily be dealt with in code.

The mic input is mono accepts all types of standard elements as it already supplies any power them might need. Piezo, dynamic, electret microphone, or even a small speaker work fine.

The line and microphone inputs are AC coupled, meaning a constant level will not be transmitted. This means you can't send CV into it like you could into an analogRead pin.

There's also a line output on the top header. This will put out the same audio as what you've been hearing on your headphones. You can control the volumes of the line and headphone out separately.

Here's the pdf for the breadboard setup.