To adjust the audio level on part of an audio segment:
The Effects Ribbon for audio effects is displayed.
The toolbar displays the Gain features.
Another key frame is automatically created at the gain level selected.
The audio level is adjusted to the new gain.
You can click and drag the keyframes in the segment to adjust the audio level. Holding Shift + dragging constrains the adjustment vertically or horizontally. Setting the gain slider to zero brings the audio level back to the segment default.