Transitions on the audio tracks
A quick way to create a fade at the beginning or end of an audio clip is simply to add a Fade transition as you would for a video clip.
To get a cross-fade between two adjacent audio clips, simply drop a Dissolve transition onto the Timeline track between them. (Actually, any transition type other than Fade has the same effect, but Dissolve gives the best visual reminder of a cross-fade.)
Transitions on the audio track: Placed at the beginning of a clip, the Fade transition fades in; placed at the end, it fades out. The Dissolve transition creates a cross-fade between adjacent clips.