When working on audio and attaching it to a previously created video timeline, the audio track seems to interlock with the video. Thus, if I try to move the audio back to start later, the video track goes back too, leaving a gap in the timeline. Similarly, if I add an audio segment, the video scoots to the end of the audio segment and must be dragged back by using "select events to End." The results are not always satisfying. How can I decouple audio from video while working on the one but not the other? The manual provides no help and on-line help is silent on this score as well.