Marisu,
Do not have any video track active, just one of you audio only cuts
park at the head of the audio cut and press T for matchframe
the clip in your source viewer is what you want, change edit mode to video only and add over existing video
This works for me all the time
LS
Larry Sherwood
Sherwood Post Production
Austin, Texas
512 219-8721
larry@sherwoodpost.com