Following installation, it is recommended to check the Audio Preferences within GarageBand. From the top menu, Preferences can be selected within the GarageBand menu, or by using the keyboard shortcut ( Command + , ).
If an audio interface is connected to the iMac, it can be selected from the drop down menu by choosing external speakers. Otherwise, select internal built-in speakers. For an iPad and iPhone, the built-in microphone and speakers are selected.