I agree about Ventus and Ansem SoD, but the reason for the change from Mickey's normal speech patterns still hasn't been explained. Also, how would Mickey know that Sora was the chosen one? Additionally, Kingdom Hearts is implied to have some degree of sentience (as seen in the fact that it chose Sora to open the door), and didn't King Mickey say that Kingdom Hearts was calling on Sora (not with an actual voice) near the end of KH2? Besides, it doesn't mean that Kingdom Hearts has a voice per se, since Sora was in the dive to the heart it might not have been a physical voice. It could have been a voice in his head, we don't know for sure since all we saw was text on the screen.
The explanation can be surprisingly easy if one gives Mickey the benefit of the doubt that he can speak in a more "normal, non-Mickey"-way when special conditions arise. I bet if Bret Iwan would actually do the stuff we see there in Mickey's voice there would be no confusion anymore at all.
This "mystery speech" was actually used by Mickey again in Coded when he alluded to Data-Sora gaining a heart (a new power!):
(A flash of light fills the screen. When it fades, the damage blocks are gone, and Sora has his keyblade back!)
Sora: ...What? What just happened? This is... my keyblade?
(Donald and Goofy get up and line up with Sora to face Pete)
Pete: What's going on here!? I thought Maleficent destroyed that thing...
Voice: It looks like a new power has emerged from inside you.
(The camera turns to the entrance, revealing the speaker)
(Mickey enters the room and stands with the party)
Sora: What's going on? What's this new power?
Mickey: I don't know all the details, but... I think something has awakened inside of you. The keyblade is tightly connected to the Power of the Heart.
Yet, an old friend told me that the Heart can't be turned into Data.
Still, as you came up all this way, we developed a Bond. It might have created something new, something that goes beyond data. I'm sure it's a unique new power that you obtained through the bonds with your friends across the worlds.
I don't think it's too off to connect Mickey with that Dive to the Heart speech, as in the snippet above he also uses no names and speaks more "formal".
As for Kingdom Hearts being sentient, that has been hinted also by Xigbar in Days, but that still doesn't hint anywhere towards KH being able to speak (even telepathically).
Furthermore, KH is said to be a conglomerate of millions of Hearts, so if it could "speak" it would need to have more than a million voices.
(As I doubt that the Hearts within KH vote for a Speaker or a chancellor who speaks on their behalf, lol).
This just occurred to me. If you played KH1 in German, the difference in Mickey's speech patterns and the voice in the dive to the heart may or may not have been translated fully. Although, German is pretty similar to English (at least in my opinion), so I dunno if that vernacular would translate. Did you notice a difference?
In German, Mickey's actual voice is close to the English one and the Dive to the Heart-speech in KH 1 is certainly held more "formal" than we are used to from Mickey, more along the lines Yen Sid, Eraqus or Ansem the Wise would speak and definitely
not Ansem SoD or Ven.
Still, Mickey can
be more formal and serious if he wants to be, so I think what irritates most people is that it is really only text.
We still don't have much information on this sort of communication, so anything's a possibility. Although, since the keyblade chooses its master, saying that Riku was supposed to get the keyblade may not be as correct as saying that Terra wanted Riku to get the keyblade. We still don't know if keyblade wielders have absolute control over who receives their keyblade or if they only have partial control. Riku was the one who gave Aqua's keyblade to Kairi in KH2. Although, it is possible he got it in the RoD.
Since I haven't played any FMs, I can't say for sure, but didn't Terra's armor (I can't remember its name) have a keyblade? If he/it had a keyblade, how could Sora have his keyblade at the same time for that fight?
Riku was supposed to get the Keyblade because he's a true chosen one, chosen by one with sufficient power to "pass the power".
Sora isn't a real chosen one since he got the ability
to wield and
the Keyblade itself due to his power to connect with other hearts.
You have a misconception here, Terra didn't chose Riku to inherit his
Keyblade. The Earthshaker/Ends of the Earth is Terra's personal Keyblade and remains his.
The Keyblade meant for Riku (Kingdom Key) is a new one.
The Inheritance ceremony passes the power
(aka the ability to wield a Keyblade) to a heart making it qualified
to wield a Keyblade, not an actual Keyblade.