I wonder if it has something to do with atmospheric pressure and temperature?
There were days in Christchurch when I were growing up as a nipper when you could hear an Iroquois coming into Wigram from absolutely bloody miles away.... and then others not so much.
Many years later when taking a ride in a kiwi UH-1H (in one of the side seats up against the transmission box facing outwards) with both doors slid back I was gobsmacked at how quiet it was.
Bit of main blade noise, but mostly gas turbine and slipstream.
Maybe the same is true of a Wokka...