Zack never actually speaks in Cloud's mind. When he hears voices giving him 'hints' - like the one prodding him to ask Tifa about what happened at Nibelheim - that's Cloud's buried, true consciousness pushing him toward uncovering the truth about himself. His real self was buried deep, and moments like that were when it pushed to the surface.

At other times, Sephiroth speaks into Cloud's mind - "The blood of the Ancients courses through my veins..." and the likes of that.

You can usually tell which voice it is - when everything falls quiet, or the "Who are You?" music plays, it's Cloud's buried consciousness. When the screen flashes white and the sinister wind noise is audible, it's Sephiroth.

Once Cloud's got his act together again, both voices are gone: He's beaten the psychological influence of Jenova's cells, so his real personality's not in hiding anymore, and Sephiroth's also no longer trying to mislead or influence him.