Working with sound in Sony Vegas 9 is pretty easy. You can change the sound in two ways. One way is to adjust the sound directly on the Vegas track. Another way is to create a volume envelope at particular points in the audio track to adjust the volume of those particular points.
To adjust the volume of a track, you move the mouse over to that tracks volume control and go left or right.
"Change Volume And Insert Volume Envelope In Sony Vegas 9"