Adding Sound Effects or Sound Recording to a Video Clip
You can add special sound effects to a clip from a pre-selected list of common sound effects that iMovie provides. Or you can record sound on you computer and add that directly to the iMovie Viewer Timeline as an audio clip.
To add sound effects, click on the Audio button in the button bar at the bottom of the Shelf area on the right.
In the window that appears, you'll see a box listing different sound effects(each of which has a blue icon).
Select a sound effect you want, such as Standard Sound Effects...Hard Rain, and then click on the large play arrow below to hear the sound effect.
To add the sound effect to the Viewer Timeline, click on the blue icon for the sound effect you want, hold down your mouse button, drag the sound effect down to the Viewer Timeline and position it in the audio tracks below the clip to which you want to add it. Release your mouse button.
The audio will be added as a new audio track for your clip.
(you also can add the sound effect by positioning the Playhead in the Timeline Viewer at a point where you want the sound effect to begin, and then selecting the blue icon for the sound effect and clicking on the Place at Playhead button)
You now can move the Playhead in the Timeline Viewer to the beginning of the sound effect audio clip and click on the Play button to see your movie play with the sound effect added to the video.
Each sound effect has a pre-set duration, but once it's in the Timeline Viewer you can edit the audio clip to eliminate some of it. Click on the audio clip and move the Playhead to a point in the audio track. Then in the menu at the top select Edit...Split Selected Audio Clip at Playhead. Now click on the segment of audio you don't want and press the Delete key on your keyboard to eliminate it. Repeat the process to edit other segments of the audio.
You also can change the audio levels within a sound effect audio clip just as you would with any other audio clip (or audio that is embedded in a video clip). See the section on Adjusting Audio.
If you want to remove the sound effect, click on the audio clip to highlight it and press the Delete key on your keyboard.
Recording Audio into iMovie
if you have a built-in microphone for your computer or a separate microphone you can plug into the audio input jack on your computer, you can use the Audio section of iMovie to record sound, such as narration, to add to your movie. The sound will be added directly to an audio track in the Timeline as you record.
To do this, connect your microphone into the computer (or just use a built-in microphone if your computer has one). Notice that the level of the sound picked up by the microphone is displayed in a bar just below the Place at Playhead button.
Position the Playhead at a point on your Timeline where you want to insert the audio.
Press the red record button to the right to begin recording.
A new audio clip will appear on one of the audio tracks in iMovie, with the sound you're recording automatically inserted into that clip. The clip will continue to increase in size as long as you're recording.
To stop recording, press the red record button again.
You then can change the audio levels of our recorded clip just as you would with any other audio clip (or audio that is embedded in a video clip). See the section on Adjusting Audio.