Place the cursor at the leading edge of the narration. Use <ctrl-alt-right> or <ctrl-alt-left> for accuracy, or use the buttons under the preview window timeline.
Split the music track. Do the same at the trailing edge of narration.
Now when the audio track is clicked in the newly split clip, the section that you want to adjust will be highlighted. If you want to expand that area, click the small icon at the far right that has red and blue brackets. Drag the blue line to the desired levels. The expanded waveform, with level dips programmed, will look similar to this...

Or use the ducker feature to do this automatically. The adjustments can be found under OPTIONS | AUDIO tab. Right-click on the music track and click BACKGROUND SOUND to enable the feature. Takes some practice, but works very well.