To delete one of two videos on iMovie

Put your cursor on the audio clip and, while pressing down, pull the clip straight down to a lower audio well until its coupling pulls off from the audio clip above it. Then you can delete the video clip without also deleting the audio clip. You may need to pull the audio clip down farther than one audio well. You should be able to see the coupling pull off, although it is not shown in your screen shot, probably because the audio clip is attached farther down the timeline.

You might also simply be able to select the audio clip, then do an Edit/Cut to remove it from the timeline, then delete the video clip, then Edit/Paste the audio clip back in. That should work, although I don’t have my Mac handy right now to try it.

{EDIT} If you do the second procedure above, before deleting the lower video clip you first may need to slide the upper video clip down the timeline and drop it into the main time line, since it may be attached to the video clip below it. If it is attached, then both video clips will delete when you delete the bottom one.

If anything goes wrong you can always do Edit/Undo one or more times to get back to where you were.

— Rich