I'm pretty sure the song is called "Prologue" in most of the OSTs.

The amount of mislabeled stuff on YouTube, particularly for IV, VII, VIII, and X, is making looking for all of them quite difficult though. The fact that YouTube seems to search for "prelude" as a synonymous match doesn't help either, since as other people pointed out it's the name of the arpeggio that plays in all FF games as well.