You can add an envelope to a track where you will see a horizontal line that represents the volume level. You can move the line up and down to increase and decrease the volume.
Right-click on the track and choose “Toggle volume visible”. You will see what looks like a new track with the volume envelope inserted. You can hit the “V” key to toggle its visibility.
You can also move the volume line into the track itself by choosing “Move to medial Lane”.
You can also add edit points to the line by right-clicking on the line and choosing “Create new point”. You can then drag the points around to adjust the volume of different sections of the track.