If you're using an older version of iMovie you might find yourself wanting to double click a clip to add an audio fade in using iMovie and then be shocked that it doesn't work. So shocked in fact you might miss how to add an audio fade in completely. I can't believe how easy it was but that I only found it after tearing my hear out (figuratively). It was right in front of me all along.
With iMovie open, drop in your audio clip. To add a fade in, just position you mouse over the left edge of the audio clip. You will see a little ball (screenshot above), just drag it to the right to create an audio fade in using iMovie.