You can adjust the overall volume of the audio in a video clip. Or you can make the audio level change many times within a single clip.
First check the Edit Volume box at the bottom below the Timeline.
Then click on a clip in the Timeline to highlight it.
Make sure the audio sound box to the far right of the track for the clip is checked (the sound boxes are underneath a tiny speaker icon). An unchecked sound box mutes the audio.
And make sure the sound level bar to the right of the Play button is dragged far enough to the right so you can hear the audio (this bar only ajusts the playback volume of the sound you hear in the computer, not the audio level of the clip itself).
To change the overall audio level of the clip, click on the audio volume slider tool in the lower right below the Viewer.
Drag your mouse to the left to reduce the audio or to the right to increase the audio. Notice that a thin purple horizontal line on the clip shifts up or down as you adjust the audio.
You also can use this purple horizontal line to adjust the audio to different levels at different places within a clip.
Click on the purple horizontal line at any point and drag the line up to increase the audio from that point on, or drag the line down to decrease the audio. A small circle will be inserted on the line at the point you click on it.
Then click on the purple horizontal line at another point on the clip and again drag the audio level up or down from that point on.
In professional audio editing applications, this process is called "editing the envelope."
If you want to clear a point at which you've adjusted the audio, click on the small circle for that point, and in the menu at the top select Edit...Clear.