My reasoning? If the atria are fibrillating, they aren't generating high enough pressures in the chambers to force much blood through the stenotic valve. Since the turbulent flow of blood is what produces the murmur, perhaps not enough blood is forced through the valve with each atrial "contraction" to generate an audible murmur?
Someone can correct me if my reasoning is wrong, but what's with all the cardiac questions coming out of left field?