Just what I’ve been needing. I’m fixing large amplitude changes in church recordings from switching to/from the ceiling mic to pick up voices from the congregation. I generally have to amplify the region, but mic picks up everything, so up comes the b/g noise along with the voice. I use Fade by dB to ramp the ends so they match the original level before I boosted it. Doing this with the envelope tool was all guesswork.
Put it in the Audacity Plug-ins folder, then restart Audacity and it will appear in the Effect menu (below “the line”).
On Windows computers, this is usually under “Program Files”. On Mac OS X, it is usually under “Applications”