To adjust the rate:
In your iMovie project, select a video clip, audio clip, or background music icon.
Click the Audio Adjustments button (looks like a speaker with a waveform coming out of it) in the iMovie toolbar to open the Audio Adjustments window.
To set the rate at which the sound fades in, select Fade In: Manual, and then drag the slider to the left to make it fade in faster, or to the right to make it fade in more slowly.
To set the rate at which the sound fades out, select Fade Out: Manual, and then drag the slider to the left to make it fade out faster, or to the right to make it fade out more slowly.
Click Done.
