Ahh! I see, well I guess that makes sense. Though w may never actually know the circumstances of why he heard the voice. For all we know, Sora was just passed out after sniffing too many paupu leaves....You're remembering correctly; it should have gone to Riku, but he let himself fall into the darkness instead. What I meant was that the Keyblade hadn't presented itself to anyone yet at that point, either Sora or Riku. So, if Mickey was calling out to a Keyblade wielder (the one he mentions in the letter), neither of them would have fit the bill. That's why I am thinking it had something to do with Ven's Keyblade already being there; although dormant, his is technically the only one left with its wielder at that point in the series. That's just my speculation own speculation, though.
It still makes sense that it was Mickey, though, given what he says about opening the door and, at the end, the door to light. I guess it's just a matter of why his voice spoke to Sora's heart instead of Riku's, which was where I got the idea about Ventus.