The earliest this can be unlocked is in the mission 'Mittelwerk Facility', at the start and to your left you will notice a guard tower. Move slightly to the right of your starting location then look up to it, you will see a guard who will stop and light a cigarette.
When he stops aim your crosshair on him, a sound indicator will appear in the top right hand side of your screen when the speakers start to sound off. When you see this take the shot, the loud sound of the speakers will cover up the shot of your sniper rifle. Job done.