Working with Video Media Resources
When added to a project, a video resource will by default be set at its full run length. If the video has audio, that audio will also automatically be added and will always be set to have the same start and end times as the video resource.
If you extend the 'length' of a video in the timeline, this will cause the video (and audio) to loop.
The times at which the video loops end/begin are shown by this mark: 