i dont think that the voice in the awakingning is anyone...
at the time of awakinging, mickey wasnt aware of sora's exsistance. yes, he probly did know there was a new keyblade apprentice on a "backwater" world (in the famous words of Donald duck xP), but more the likely never knew WHO it was. so it probly not him
mickey was talking to riku...he wasn at the door too, right. there hasnt been an awaking scene for riku yet (sad but tru) so mickey talkin to riku (at the door) isnt the same as the disembodyed voice talkin with sora
now, chonologicly (in the kh verese) ven was connected with sora at the time of the awakinging. So it could possibly be him- guiding sora along the path of light... to bad nomu never thought of that, cuz, chonologicly, IRL, kh2 and above was never planed out when kh1 was produced... pity, really ^^
mickey did talk to sora in AN awakinnig...well, data sora, any way. this is the only awakinging where the voice has an actual...well, voice ^^...the only thing about it is that mickey is talking to holo-sora- so it dont count D:
i personally believe it is Ven who is conversing with sora (and as a consequence-Roxas), because ven is apart of sora's own heart, aqnd is the one canidate that makes since STORYWISE (again, never thought of before duing kh1...) but, until nomura actually gets out of his 3 story fudal mansion on top of mt fuji and says "the voice in the awakinging is *insert ppl here*, it's any bodys guess...
other possible combinations are
aqua
terra (?)
Yensid
merlin
pluto (no really 8D)
it could be nomura himself...you can never be sure with that guy ^^