PTK and others are partially correct. You can silence a rifle/pistol firing a supersonic round as far as the noise from gas escape goes. But a supersonic bullet creates a "sonic boom", that is heard downrange as a sharp crack. That is a result of the bullet's speed and there is no way any "silencer" attached to the rifle can quiet it unless it allows enough gas escape before bullet exit to reduce the velocity below the speed of sound.
The old Maxim silencer, probably the best (and most expensive) ever made, would reduce the sound of M1903 rifle to the click of the firing pin IF the bullet was fired directly into a backstop. But once the rifle is fired down range, that "crrrraaaaaaaaccccccccckkkkkkk" can be easily heard.
That is why folks say you can't silence a rifle firing bullets at high velocity, not that you can't silence the noise from the precursor wave and the escape of gas from the muzzle.