Hollywood and other games have created a HUGE false impression about “stealth kills”. In real life, the purpose of a “Stealth Kill” is primarily using stealth to get into a position from which to launch a killing attack on the enemy without giving him any chance to hurt you in return. Done right, this IS rather quieter than a stand-up fight, but it still unavoidably makes a fair amount of noise. So you’re only going to remain undetected by other enemies if you make the kill out of earshot.
There are several sources of noise when killing somebody with a knife. These are:
Impact of the Blow: If you attack through bone (either the ribcage or the skull), you have to hit hard enough to get through. At the very least, this sounds like a good punch landing in a fistfight. If you pull the guy’s head back and stab down through the base of his throat, while the knife going in doesn’t make much noise, there’s still the thump of you pulling his back into your chest.
Victim Noise: Other than going into the base of the skull, there’s no instant death here. Even when stabbed down through the aorta and into the heart, it will still take some seconds for the victim to bleed into unconsciousness. During this time, the victim will at least gurgle fairly loudly, grunt, and maybe cry out in pain if you left that an option. Besides this, the victim will likely thrash about some (even when skull-stabbed) and will likely drop whatever he was holding at the time. There’s not much you can do about that because you’re usually using both hands to hold his head just right and stab, so you can’t control the guy’s arms.
Vertical-to-Horizontal Noise: If your victim was standing up, he’s now going down. If you let him fall naturally, he’ll make a lot of noise. If you lower him as best you can (realizing he’ll be thrashing at least some, maybe a lot), you still make noise although this depends somewhat on what the victim was wearing and holding, and what the ground surface is.
Bottom line is, unless there’s something to keep others from hearing all this, you’re going to be heard. Simple as that. What makes others not hear all this is distance, intervening walls, and ambient noise. If you’re in the same room with other enemies, you’re busted. If you’re in the next room, MAYBE you can get away with it depending on what the intervening wall is and how loud other activities are at the time. The quieter it is, like if everybody’s asleep, the more noticeable your noise will be. So if you’re in a busy market, you could well get away with it just out of sight around a corner because the noise will either blend in with the ambient or be covered by it.