Premiere Rush has minimal audio control. You can make some refinements, but it doesn't have fade in or out transisitons:
As a workaround, you could try selecting the audio, placing the playhead at a location where you can use the Scissors to split it, and appling a volume adjustment to reduce the volume for that portion of the audio. You could repeat that process to simulate a fade in or out.
Or you could open your Premiere Rush project in Premiere Pro and apply audio transitions to fade in and out the audio there.