"Suddenly Voiced" is when a Voiceless character, a character who doesn't have a voice actor before but is implied to speak in-universe, eventually gains one. "Suddenly Speaking" is when a character who is canonically mute, or at least communicates in noises like animal sounds or zombie moans, speaks in-universe. Like when Big Mickey in Thomas and Friends, who didn't speak (or even have a face) prior to "New Crane on the Docks" not only starts talking as of that episode, but suddenly has a face.