I don't know if Envy could pull it off well enough to fool Winry close up, though. Automail has to have a sound - hydraulics and steel - and metal and oil have a particular odor.

Now...if it was from a distance...close enough Winry could see him and hear him, but not that close, it would work. And I'm sure there are terrible, evil things Envy could do from a close distance. XD