Of course there are wonders in the world, some might respond to the previous chapters, but that's not the same thing as saying that acts contrary to nature can occur. It might be wonderful that human beings can emulate singing from their rear ends (see Book XIV for that tidbit), but that is just a strange part of our nature. And strange is not the same thing as against. We cannot, for example, stick our hands in a fire and come away unburnt--that would be contrary to nature.
Augustine replies that God made nature, and can adjust it as need wishes. We know that He has done this once already when He cursed the earth and changed it to its present state. He can, if and when He wishes, likewise change us so that we are other than we are now, while still being the same people.