Doesn't Sephiroth control him directly when Cloud gives up the Black Materia?
The voice being referred to here is the one when the strange music plays. It's heard during THAT odd scene at the Honeybee Inn, at the Mako reactor, at lower Junon, etc. I doubt it would be Sephiroth telling Cloud to ask Tifa why the two didn't spend more time together when he was at Nibelheim.