iMovie allows you to easily add sound to your video clips and it allows you to add preset sound effects, or to use audio from CD.
To add audio or sound effects, you will use the timeline. The timeline is at the bottom of the screen and is accessed by clicking the clock icon. On the same toolbar, clicking the eye icon brings up the clip viewer. To add audio, click on the clock so that the timeline is showing. The timeline has one layer for the video, and the audio that goes with it, and another two layers for audio. Next, click on Audio at the bottom of the clip shelf. This brings up the audio menu in the space where the clip shelf normally is (the upper right side of the screen).
To add a sound clip, select the sound clip from the top of the audio menu and drag and drop it into the timeline where you would like it to begin. To test, play your movie and drag and drop the sound clip to change its placement.
To add audio from a CD, first insert the CD. Once the CD is inserted, make sure the timeline is open and go to the audio menu by clicking Audio under the clip shelf. The CD tracks will be listed in the audio menu. To add the CD track to your movie, simply drag and drop the CD track into the timeline where you want the CD to begin. Play your video to hear the video with the new sound.