You can apply crossfades for audio transitions between clips. An audio fade is analogous to a video transition. For a crossfade, you add an audio transition between two adjacent audio clips on the same track. To fade in or fade out, you add a crossfade transition to either end of a single clip. Premiere Pro includes three types of crossfade: Constant Gain, Constant Power, and Exponential Fade.
For descriptions of the available crossfades, see Audio crossfade transitions.