Double-click the audio track. The clip waveform appears. If the clip has been split, inactive areas will be shaded. Drag the blue line that's in the active area.
Each time that's done a keyframe will be created. In this example the volume is raised at keyframe 1 and lowered at keyframe 2.
Or instead right-click on the audio track, click EFFECTS and apply Amplify. Move the cursor and click the small green + to create the first keyframe..
Drag the cursor to the next point. Then move the Gain slider, which will create the next keyframe...and so on. Note the PRESETS as well.
