A low obstacle (such as a wall no higher than half your height) provides cover, but only to creatures within 30 feet (6 squares) of it. The attacker can ignore the cover if he’s closer to the obstacle than his target.
i read this as:
Creatures that are within 30 feet of a wall can only be attacked normally (ie. target doesnt have cover) if the wall is shorter than half the attackers height, unless the attacker is closer to the wall than the victim.
Thus, if the victim is within 30ft of a wall which is taller than half the attackers height, they always have cover, unless the attacker is closer to the wall than the victim.
so in the case of a human with a reach weapon attacking over a low wall, the victim only has cover if the wall is taller than half the attackers height.
as i stated earlier, the wall in this case is a kobold,
and the MonterManual says a kobold is 2 to 2-1/2 feet tall,
so as long as the attacker is at least 5ft tall, the kobold does not grant soft cover to the sorcerer, and the sorcerer provokes AoO's
the only potential problem i can see is: according to the rules in the jump skill description, a small creature has a vertical reach of 4 ft, so if a kobold holds its arms straight up, does it then count as a 4ft wall, and then provide cover? (which is more than half a humans height)