Select the audio event and press V on the keyboard, or go to the Insert menu and insert an audio volume envelope. You'll see a thin blue line running through the center of the audio track. Double-click this line just before you want to fade, and again shortly after that. Small blue points appear on the line. Add two more where you want to volume to go back up to normal. Now drag the middle section of that line between the two sets of points down. This lowers the volume during that section and creates a fade down between the first pair of points and a fade up between the second pair. Add as many points as you need. The points themselves can be moved up or down to adjust volume, and sideways to adjust timing.