When you open the Audio/Video Editor, the timeline will be scaled so that an audio or video clip fits it without scrolling. That's a convenient starting point, but when editing, you may need to zoom to a specific part of the clip.
To scale a timeline:
There is also a quick way to scale the timeline to maximize a selected place and zoom out to show the entire clip.
To zoom in to a selected place:
To zoom out to show the entire clip: