Audio problem: During the scene where he is hitting golf balls off of the ship, he is hitting an iron. For anyone who plays golf, the sound that goes along with this is that of a driver or some type of fairway metal.
Audio problem: Just after the lion takes the deer Neville was hunting at the beginning of the movie, his watch alarm goes off to signal that it's getting dark. The watch alarm in the movie is a "beep-boop" two tone. As the owner of both watches Neville is wearing, I know that the watch we get a closeup of, a Swiss Army Sportech 4000, has a single (once per second) alarm beep and the watch on his other arm, a Hamilton Khaki X-wind, does not have an alarm. (00:07:30)
Audio problem: Aside from the beginning of the movie, the generators Robert has going put out a lot of noise that we do not hear during any of the other home scenes. This would make sense for the night shots as they would be off as to not attract anyone, but nothing is heard during any other daytime scene.
Audio problem: In the beginning of the movie when Robert is chasing the animals with his car, he points his gun out of the window and you can see his dog barking. But you can see the dog's mouth is moving without making any sound. (00:04:40)
Answer: In the alternative ending Dr.Neville, Anna and Ethan are seen driving a car over a bridge that wasn't destroyed. The army would need to leave a entry/exit point, but it would have been heavily guarded until everyone died.