You want to add a fade. Hover the mouse pointer in the upper right corner of the audio event. The pointer will change to a little quarter-circle pie-slice shape. Hold the mouse button down and drag left. You'll see the end of the event turn darker with a line drawn on it that slopes down. This shows the volume gradually decreasing. Drag farther for a longer fade.

Right-mouse-button click in the faded section and click "Choose fade type" to select different types of fade curves.