This can be done, but it is a little complicated until you get the hang of it. I use it all the time.
To make things easier, go to iMovie/Preferences and make sure there is a check mark next to "Display Time as HH:MM:SS:Frames. With this checked, you can edit to the frame level - otherwise only to 1/10th of a second.
There is a command in iMovie 09 called "detach audio". To use it, select the clip in your Project pane that you would like to extract audio for. Then either click EDIT/DETACH AUDIO or you can right-click (or control-click) on the clip and select DETACH AUDIO from the contextual menu.
You will notice that the audio has been extracted from the video clip and pinned right under the video clip.
However, the audio is still there in the video clip, too. So you need to go into Inspector/Audio Adjustments and set the volume slider of the Video Clip to zero. Now you will still hear the audio, but it is coming from the extracted audio clip.
Now you are ready to replace the video frames in the clip with alternate frames - this can be either video from your events or still frames (photos) or stills extracted from elsewhere in your movie.
OK left-click your mouse and drag to form a Yellow Border to select the exact frames that you would like to remove from the original clip. You can fine tune by dragging the handles of the yellow border left and right. When you have it like you want, select EDIT/SPLIT CLIP (or right-click/SPLIT CLIP). Note the exact number of seconds and frames that this split clip contains.
You will need to bring in video (or a still) with this exact number of frames (this is so the audio will line up at the end), and drag it next to the clip you want to remove. Once it is there in your project, you can select the clip containing the frames you want to remove, and delete it.
Sounds complicated, but pretty easy once you have done it a couple of times.
