It's important to set up the input and output of the audio for your project so that you can record and hear playback correctly.
1. In an open GarageBand session, go to GarageBand --> Preferences.
2. Select Audio/MIDI in the dialogue box.
3. Choose the Audio Output and Audio Input that you want to use. Generally, it is best to use outputs and inputs of the same nature. For example, both should be built-in or both should be external.
