The envelope tool is found on the Audacity Control Toolbar.

Use this tool to manually adjust the volume of any portion of your sample without actually changing the sound file (until it is exported). The envelope tool is especially useful for editing interviews and is also the tool you will use to bring sound “up” or take it “down” in your intro.

Turn the envelope tool “on” and you’ll see blue borders around your track. Click the blue line to add a handle (it will look like a white square point). When you have more than one handle, you can change the volume of the sound between the two handles. Widen to increase volume and narrow to reduce volume. To remove a handle, click-and-drag it off the track.

Source:Audacity Help. Watch video tutorial; Lynda.com tutorial.