The 3 different rifles have 3 different reports for 3 very different reasons. The rifles you are trying to compare are like comparing apples, oranges and kiwi.
Lets start with the M-16/AR-15. The only difference between the AR-15 and the M-16 is the fact that one is Semi-Automatic the other has a fully Automatic selection on the fire selector. Other than that the rifles are the same
The AR-15/M-16 both fire the 5.56 NATO, (.223 Remington) cartridge. Despite its .22 Caliber projectile, this is actually quite a powerful cartridge, and a very accurate one. This cartridge also operates at near twice the pressure that the 7.62x39 cartridge does. Because of this, there is more of a "Snap" or high pitched pop that your ear hears than that of the 7.62x39 Cartridge.
The other two folks that mentioned the flash hider and or a recoil suppressor are incorrect as to increasing the noise the rifle makes. The job of those devices is to as their names imply control recoil and muzzle flash, then direct the gasses away from the shooter. This makes the rifles easier so shoot and control, additionally since these are designed for combat, it makes follow-up shots easier for the soldier, with the respect of being able to get the weapon back on its intended target. Just as a note this applies to all of the rifles you are seeking an answer for.
Now on to the AK-47. The SKS is NOT a relative of this rifle dispite what you may have been told. They are two completely different rifles, with two completely different design patterns to fire the same bullet. They fire the same bullet, and that is where the similarities end. The gas operation systems are different, as are many of the internal mechanical parts.
To the eye the rifles may look like relatives only because they share some design similarities in their stock design toward the front end of the rifle. But again, that is where the similarities end.
The 7.62x39 cartridge when fired makes more of a thud than a snap. This is because it is a low pressure cartridge, has a larger diameter bullet, and a lower powder charge.
For lack of better terms the sounds are like comparing .357 Magnum and .38 Special. Two different cartridges operating at two completely different pressure zones.
The projectile for the 7.62x39 is nearly double the weight of the .223 Remington, and is traveling nearly 1100 Feet Per Second slower than the .223. Being that the projectile is also twice the weight, this also means the recoil the shooter must absorb is more significant.
The all of the rifles do share one similarity, in the respect that they are all gas operated actions. This means there is a port either in the barrel or the action that collects a portion of the gasses and cycles the action through an actuating piston to eject and chamber the next cartridge for firing. But even having said this, the three systems are different, with respect to the AR-15/M16, the SKS and the AK-47.
It should be noted the Gas System in an AR-15 shares the same system with the M-16, because they are the same rifle.
Now having given you this information, if you seek a more indepth examination, I would suggest getting a book or two from your local library or any one of a million sources on the web.
Good Luck and Happy Shooting.