In June, a fan posted on KH13 with a video of a Facebook convo with Ben Diskin (the voice of Young Xehanort in the teaser trailer), where Diskin was nice enough to send a voice message with the lines from the teaser trailer in Young Xehanort's voice. That topic is here:
http://kh13.com/forum/topic/75067-i...diskin-doing-the-kh3-trailer-lines/?p=1433083
Curiously, that user removed their YouTube upload of this video today. They also created a topic on the forum about this rumor (here:
http://kh13.com/forum/topic/77701-l...llegedly-being-worked-on-according-to-reddit/), where they don't see fit to mention that Diskin recorded the same lines for them back in June.
I don't have proof since the original upload is gone, but I'm 90% sure that this is a rumor made up by this guy