Voice-over involves the use of spoken audio text along with the video content. This spoken text does not come from an actual actor on the video screen but is typically recorded after the video is produced. The audio content may or may not relate to the actual images on the screen.
Hence, the original audio is normally lowered or muted completely to allow for the audience to hear the voice-over more clearly. This technique is often used in documentary films or productions since the subject matter does not involve live actors.