Treat it the same way you would treat it if Timmy looked you in the eye and said "I won't do that." He is hearing you and refusing to obey - it is plain disobedience. If one of my kids was playing a game or reading a book and they pulled this trick, and I was pretty confident that they really did hear, I'd probably take their game or book away for the rest of the day. Or go over, turn the tv off or get in front of them where they can't ignore me, make them put their book down, and tell them, "do you know what you are doing? You are trying to disobey by pretending not to hear. Now you get to help carry the laundry and then do the dishes on top of it." Expect obedience and never tolerate disobedience.
I like Chrys' idea, and that should be a general parenting pattern. Our kids should love the sound of our voices. To them it should trigger thoughts of encouragement, fun, listening to their problems, words of love and affection.
Still, even so, they might try this trick. Kids test those boundaries; they have to, because they do not know how firm or what parameters the "obey" rule has. To discover what "obey" looks like, they have to run little experiments. It's up to us to make "obey" a very consistent concept; deviations not tolerated. Never say "oh, well, I'll just do it myself." That might be easier in the short run, but encourages a pattern of disobedience that will bite you hard in the long run. Once the order is given (and a request from a parent is an order), it should be obeyed.