iMovie HD: CONTROLLING SOUND EFFECTS VOLUMES

If you import sound effects (or use the built-in sound effects), you can control the volume of each sound effect individually. Just click directly on the sound effect you want to adjust (in the Timeline Viewer), then move the Volume slider (that appears to the right of the Clip field near the bottom-left side of the iMovie HD window) to the level you'd like the effect to play. iMovie will keep track of each sound effect's individual volume and adjust them accordingly as your movie plays.