Yes, that is his voice in the film’s teaser trailer. Although this has been roundly confirmed, particularly after a brief period of time when everyone was convinced that the voice belonged to Benedict Cumberbatch—who isn’t even in the film! probably!—it’s nice to hear Serkis confirm it. He even added another tidbit to this confirmation: The voice he used in the teaser is the actual voice of his character. Creepy.
Alright you haters, I know a thing or two about a thing or two, and I was right.