For precise audio editing, like creating fades and crecendos in your tracks, you should create keyframes in your audio clip using the Pen Tool.

Open an audio track by double-clicking on it in the Timeline, which will open up an audio editing box in the Viewer above.

Then click on the small icon that looks like a mountain on the lower left of the Timeline.

Then activate the Pen Tool by clicking on the icon looks like the tip of a pen on the Tool Pallette (or press the P key on your keyboard). Your cursor will change to a pen-like icon when you move it over the horizontal purple audio level line in the Viewer.

With the Pen Tool, you create keyframes by clicking the red horizontal audio level line in the Viewer at the points where you want the audio to change.

Then move the Pen Tool over a keyframe and it will change to a cross. Click, hold down your mouse button and drag the keyframe point higher to increase the audio, or drag it lower to decrease the audio at that point. Notice that the red line will slope up or down to the other keyframes you've created with your Pen Tool, so the audio will fade in or out between the keyframes.

You also can drag a keyframe to the left or right using the Pen Tool.

To delete a keyframe, click on it and press the Control Key. In the pop-up box that appears, select clear.

Using the Pen Tool to Fade Video In or Out

You also can use the Pen tool to create similar fades in a video clip. Thus if you have two video clips stacked on top of each other in the Timeline, you can have the top clip fade out to reveal the video clip underneath.

To do this double click on a video clip in the Timeline so it appears in the Viewer.

Then click on the small icon that looks like a mountain on the lower left of the Timeline.

You'll see a black horizontal line appear at the top of the video clip in the Timeline.

Select the Pen tool as described above, click on the black overlay line to create keyframes in the video clip, and then drag a keyframe down to make the clip less opaque - fading out to display any clip underneath.