Very few kills are 'silent'. But somehow I don't see CoD forcing people to wait 30 seconds whilst their character writhes on the floor in agony, their movements getting smaller and less effective as they bleed out...
Also, if you can hold someone's mouth for a couple of seconds, apparently you can cut the nerves in the spinal column with a knife between vertebrae. Just as silent, as the recently stabbed can no longer move or breathe.
(Disclaimer one: Read it in a few books, I'm not some kind of knife-wielding maniac)
(Disclaimer two: Said books were fictional. This may be bullshit, though it sounds like it would work)