In all Editing Modes you can adjust the volume of a video clip.
Select the clip whose volume you want to edit then click on "Edit".
The editing viewer will open on the left. Select the "Volume" Icon.
To adjust the volume drag the volume slider. You can even boost your audio over 100%. Click on "Done editing" to apply the new audio volume.
To mute the clip, click on the "Mute" checkbox. You can also fade audio in and out by dragging the "Fade In" and "Fade Out" sliders.
In Timeline Mode, you can adjust the volume of the clip by changing the height of the track