 The planned route required a steep descent into Madang and, although the aircraft was descending steeply, the propellers were left at their cruise setting of 900 rpm, causing the airspeed to increase.Neither pilot noticed the airspeed increasing towards the maximum operating speed (VMO); as they were "distracted by the weather".When the aircraft reached VMO as it passed through 10,500 ft, with a rate of descent between 3,500 and 4,200 ft per minute, the VMO overspeed warning sounded.