iMovie's transitions library enables you to apply elegant cross-fade effects to your project's video and title tracks, but the program's audio tracks are handled a little differently. When audio is added into iMovie, fade-in and fade-out effects are automatically applied to each sound clip. But if you don't want to use iMovie's auto-fade settings or you'd rather not use audio fade effects at all, you'll need to make a few simple adjustments to each of your project's audio files.
Launch iMovie. Select one of your project's from iMovie's "Project Library" window.
Right-click on any click in your project, and then select the "Audio Adjustments" option from the menu box that pops up. The "Audio Adjustments" menu will appear.
Check the "Manual" options next to the Audio Adjustments menu's "Fade In" and "Fade Out" headings.
Drag the "Fade In's" slider to the left or right to specify your desired duration for iMovie's manual audio fade ins. Drag the slider to the right to increase the length of the fade in, or to the right to decrease it. Set the "Fade Out" slider to the desired manual fade out. To disable audio fading altogether, drag both sliders to "0."
Click on the "Audio Adjustments" menu's "Done" option to exit the menu and store your changes.