Nobody knows. It was never confirmed. We can keep guessing and making theories... but until an interview or a game tells us who it really is, it will remain to be a mystery.
Yeah it was confirmed to be Mickey in the KH1 Ultimania. Hard luck finding it again, but when it was confirmed someone did post the scan of it with the snippet proving it. Mickey is also scene doing this to Riku in KH1:FM and CoM and says he can talk to people through their hearts so, it's easy to see how.
Also, I know what you mean about the DtL. It being basically just a reverse Dark Corridor at the end of KH2 is what convinced many people to not believe that was actually the DtL and was more that Sora was mistaken. But, to be fair, no one ever said Sora HAD to open the DtL, they just said that he would.
I think "The Door" (which the voice brings up in the opening Awakening) and the "Door the Light" which the voice brings up at the end of the game might actually be different things. When the door closes Mickey says "Remember Sora, there will always be a door to the light" and he says this as if explaining how MICKEY would be able to leave the Realm of Darkness. The DtL that Sora opens/uses at the end of KH2 fits very much in line with the way Mickey talks about the DtL at the end of KH1, but it doesn't really seem as important as "The Door" that he talks about in the awakening.
tl ; dr I believe "The Door" is something else (probably KH2), while the Door to the Light was just a reminder to Sora that, even if he was trapped in the darkness, he'd be able to leave somehow.
