Wiktionary
audio commentary
n. An audio track on a video recording that is timed to the footage of a film that typically consists of one or more people commenting about the film being shown.
Wikipedia
Audio commentary
On disc-based video formats, an audio commentary is an additional audio track consisting of a lecture or comments by one or more speakers, that plays in real time with video. Commentaries can be serious or entertaining in nature, and can add information which otherwise would not be disclosed to audience members.