http://www.youtube.com/watch?v=y54aLcC3G74&feature=related
Boom!
A railgun is not a gauss gun.
While both types use magnetic fields to accelrate the bullet, they are using different systems to do so.
Also the bullet used within those test have a weight of more then 3kg. Comparing that to an M24 sniper rifle bullet has a maximum weight of 11,66g including 3g of propellant. So lets say a gauss bullet would be arround 10g. So it is 300 times the amount of mass. Also the bullet will be greater, and propably have a worse aerodynamic, therefore you need even more energy to accelrate it to those speeds. The current railgun prototypes build for ships have a muzzle velocity of about Mach10.
The Gauss-Rifle in Crysis2 will propably not fire a bullet at this speed, since the speed is decreased by the shorter barrel.
Sure it would make a sound, but I don't think that the sound would be any louder then the sound of a silenced sniper-rifle.