To Change the Volume of an Audio Track:
You may want to make one track of your audio play softer or louder than another.
To lower the volume of a certain track, go to the left of the screen, below the name of the track, where you'll see a button labeled waveform.
Click and hold down this button to see several other options for that track, including volume. Select that.
When the volume option is selected, you will see a black line across your track. If you take the Grabber tool and click on the line, you can create an insertion point, which you then can drag up or down to create a higher or lower volume level from that point on in the track.
As you drag the point up or down, you'll see the db or decibel level displayed, from 0 (for the highest volume) to -96 (for the lowest volume)
Create multiple insertion points along the line and drag the audio level line up or down at each to create fades or crescendos.
Thus if you click a point far away from the beginning of the track at a much lower point, you'll create an angled black line going all the way down. This will do the same to your audio - creating a gradual fade.
To delete an insertion point, hold down the option key and click on that point.
To delete many points, pick the Selector tool, click on the track, hold down your mouse button and drag across your track. Then press the delete key on your keyboard.
This will delete all your insertion points (note that this will not delete your audio track, because we are in volume mode).
When you're done adjusting the audio volume levels and want to return to editing, such as deleting or moving segments of a track, be sure to click on the button now labeled volume, and select waveform instead.