In iMovie for Mac, first, create a new project and import your video clips and audio files. Drag and drop the audio elements onto the timeline, aligning them with the appropriate part of the video. To edit the audio properties, click on the audio clip and use the built-in audio tools to adjust volume, fade-in or fade-out, and trim. You can also apply audio effects and filters if desired. Once you have edited the audio to your satisfaction, export the final video with the edited music and sound effects.