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When you have most or all of your research done, images and sound collected, and original content produced, you can start to edit everything together. You can do the editing of your project in either iMovie, Final Cut Pro or Adobe Premiere Pro. You can work on the audio directly in the program you are using to edit or work on the sound in Garageband or Audacity and then import it into your editing program at the end.
If you want more control over your audio editing or you want to create your own soundtrack, try GarageBand. It is easy to learn and includes lots of royalty-free loops and sound effects for you to experiment with. Check out this LinkedIn Learning in depth tutorial for an extensive overview of using GarageBand. The video below provides a quick introduction to the basic features of GarageBand.