How do I adjust the volume of a clip?

Apply the "volume" audio effect.

A line will show up in the waveform of the clip.

If you double click any point on that line, a new keyframe will be added in which you can adjust the level.

If you then double-click on the keyframe (it appears as a square on the line) you can then manually type in the desired level or shift the keyframe to an exact time.