THE JUNGLE IS STILL THERE. That much can be said. Flying over the floodplains, swamps and foothills of the Lower Sepik to the sleepy river villages of Angoram or Ambunti, one is still amazed and somewhat chilled by the immensity and seeming impenetrability of it all. But the river villages themselves are disappointing. Trade stores of corrugated iron, full of Japanese canned mackerel and cheap Hong Kong fabrics, cater to Sepikers dressed in Bermuda shorts, thongs and singlets. Twelve-transistor Panasonic portable radios replace the traditional drums and flutes, and the Jefferson Airplane floats