Rinoa's splits have no mouths!! Now, we can safely assume that Ellone (Rinoa) is actually mute and communicates telepathically by making Squall and friends think they hear her voice and see her (non-existent) mouth move. Because of this, we know that Ultimecia's speech impediment (The C=K theory) is due to Rinoa's (Ellone's) mind eventually deteriorating over the many years.
OMG, I wonder how people didn't notice this!?! *Sarcasm to the extreme*
