Adding a Commentary:

Before you begin to record a voice-over for your video, ensure that the input sound on the computer is set to either Built-in Mic, if the computer's own microphone is to be used, or to External Mic, if an external mic has been connected to the computer for the recording.

The Sound Controls can be found by holding down the Apple menu and selecting Control Panels, and then Sound from the sub-menu.

The Built-in Mic looks like a small hole at the top of the computer. A voice-over can be recorded by speaking directly into it and adding the recording to the timeline.