The mentioning of the Ancients made him recall something what Sephiroth said, because later Sephiroth says it during 'Clouds past'.

The voice is probably Sephiroth messing with his head, since he's got a grudge against Cloudy for messing up his whole grand plan and whatnot. Talking to Cloud in Junon, I feel it's reasonable to assume it's Sephiroth, since when Cloud asks Tifa about it and she avoids the question, it just makes him more curious about it. In the end all of this messing with Cloud's mind leads to a major melt down and results in Cloud giving the black materia to Sephiroth...
seems like a good revenge, and something Sephiroth would do.

well i always thought if at this point his brain has been damaged by mako.. seeing at the point where he just got out of hojo's lab his brain was very vulnerable and thats how he took on another persons memories and believed it to be his own. having another personality wouldn't that make him sort of skitzo. and if so the voices in his head are another part of him in his subconcious, maybe the real him. btw when i say a different personality im referring to the badass he is in ff7 for ps1 is not the same in crisis core and his memories as a child... in each of the latter he is no where near a badass, he is very timid and shy..

holy crap i was right on...

http://uk.faqs.ign.com/articles/698/698416p1.html