To resolve this issue:
- Quit GarageBand.
- From the Finder, choose Home from the Go menu.
- Double-click the Library folder.
- Double-click the Preferences folder.
- Find the file "com.apple.garageband.plist" and drag it to the trash.
Note: Performing these steps will reset any customized preferences in GarageBand to their default settings (such as the selected audio input and output device).