You know, after I played through it the first time, and then started a second game, I actually thought that the voice was one of the PoH's.
Probably that connected in my head because of the platforms the tutorial was on.
To this day, some part of me still thinks that it's the PoH, but Ven or Mickey being the voice also sounds likely to me.
I would rather more say Ven, because I don't believe Mickey knew that Sora was the keybearer when he first set off on his journey, at the beginning of KH1. If he had, wouldn't he have told Donald and Goofy to go straight to Sora, and Destiny Island, instead of going to Traverse Town and finding Leon, who would help them in their quest?
And if he hadn't known exactly who the keybearer was, then he wouldn't have been able to be the voice from the beginning.
Of course, this is me just speculating.