The Audacity volume envelope works just like the one I mention here:
http://taperssection.com/index.php?topic=99036.msg1319812#msg1319812After engaging the volume envelope, you should have a horizontal line (yellow, in Audacity?) that represents the baseline volume, i.e. the current state volume. Add two points
on the line for your start / end points. Then, in between, add points above or below the line to increase or decrease the volume. The number and placement of points defines the volume envelope -- slope, or curved, or jagged, or what have you. In your case, it looks like a linear fade out, so you'd have three points: one at the starting point, one at the end point of the track, and a third above the endpoint defining how much gain to add in a linear increase. It would look something like this: