Originally Posted by Translated from the Final Fantasy VII Ultimania Omega
(Page 68) As Cloud is trying to place the bomb, a mysterious voice speaks to him. Later, this same kind of mysterious voice resounds in Cloud’s head in the scene where he and a voice have a conversation, and it offers him reminders. These voices are the original Cloud, as — due to Hojo’s Sephiroth Clones experiment — Zack’s personality merged with his, creating the present day Cloud, producing a conflict of multiple personalities(—>P.11).
During the scene in the first mako reactor, what [the voice] says is it [trying] to point [Cloud] toward the memory of Sephiroth five years ago in Nibelheim’s mako reactor, as the two places were similar. However, Cloud is not able to listen to the words of the memory.