User blog comment:LuffyPirateKing/Nice one, Oda/@comment-4493594-20111202140749

So wait... Luffy can hear the voices of the Sea Kings now but why wasn't he able to hear them back then. Or the voice of the Sea King that bite off Shank's arm? Is it because of his Haki? Or...

A more far fetched theory is that Roger could hear the Sea Kings because he had that disease and was about to die. Maybe Luffy is closer to death than anyone expects, since he went through a lot of stuff that decreased his lifespan.