I've tried to listen to how they pronounce it in the movie, but it even varies among Corleone family members. I've heard Michael say "Cor-le-own" and Tom say "Core-uh-lee-knee."
I think as long as you don't pronounce it like Senator Geary did, you'll be ok.
Michael: My father's no different than any other powerful man, any man who's responsible for other people. Like a senator or a president. Kay: You know how naive you sound? Michael: Why? Kay: Senators and presidents don't have men killed... Michael: Oh... who's being naive, Kay?
