First, if you want to edit audio in ways that your video editing program won’t let you, you may find Audacity useful. This free program works on Windows and Mac and gives you lots of powerful tools for manipulating audio.
Second, if you find you would like to incorporate video from your computer screen—screencasting, or recording video of what you see on your computer as you use it, a technique used to good effect in the video at the top of this post—you have the following options:
Mac users can use Quicktime Player. All you have to do is select File > New Screen Recording, and you can import the resulting file into Imovie.
Mac and Windows users can use Screencast-O-Matic, a full-featured screencasting program. It’s only drawback for our purposes is that if you use the free version it will put a small watermark in the video with the logo for the program.
If you decide to use one of these programs and have questions, or if you would just like feedback on your project, please do not hesitate to contact me.