Does it really matter?
Am I one of like five people that was able to enjoy this without caring what was whispered at the end? It's a guessing game and while there are some interesting theories floating around, asking people what was said or what they thought was said or what you think she said... Pointless.
It is completely open to interpretation until the writer states otherwise, which to me kind of takes away from the intrigue and any smidgen of philosophy this film is still capable of injecting, because without it it's just another gore flick.