In the scene where the head scientist in Area 51(you know, the guy that played the android in Star Trek: Next Generation) shows the President and his army staff the aliens for the first time. he says that the aliens "have no vocal cords" and therefore they must communicate "by other means".

If this is the case, how come when the alien, which was brought into the base by Will Smith, is eventually killed by a shot to the head that we hear it squeal in it's death throes!!

Before this scene, there is also another when Will Smith punches the alien out and I'm sure we hear it squeal as well !