Simple method: split the audio event before and after that section, then drag the top edge of that section down. A blue line will appear representing the reduced volume level.

Better method: insert a Volume Envelope. A blue line will appear through the middle of the clip. Double-click on this line before and after the section to add "nodes". Also add two more nodes inbetween those two. you may now drag the nodes around to adjust the volume "on the fly", as it were, from one point in the clip to the next. Add as many points as necessary.