audacity

The envelope tool is used to lower the volume at a particular point in the audio track. This is useful for creating effects like fade-in and fade-outs.

Select the envelope tool from your toolbar, and you will see a blue line appear around your track.

The cursor will change into two small white arrows pointing up and down. This specifies the region in which you can place an anchor point on the edge of your track. Click and drag the edge of the waveform toward the middle.

Click other areas of the waveform to place white anchor points. Parts of the waveform that are smaller, will play the sound quieter than parts that are larger. The waveform will shrink in certain areas giving a visual representation of how the sound is being adjusted.

Adjusting the sound levels of the audio this way does not affect the quality, only the volume. While it might seem that you are affecting the quality of the waveform, rest assured that this visual representation is only affecting the loudness.

To make a fade, drag the white anchor points all the way to the middle. A curve will form allowing you to create fades.

To remove the anchor points, drag them to the left or right until they either combine with another set, or are dragged completely off the track.

Selecting another tool will turn off the envelope function. A light gray color will still appear in the background as a visual cue to how the track was affected.