If you try out the example from the documentation and swap the audio with your helicopter sound, does it work?
https://www.leadwerks.com/learn?page=API-Reference_Object_Source_SetRange
range = 1.5
--Create a window
window = Window:Create()
context = Context:Create(window)
--Load a sound
local sound = Sound:Load("Sound/heli2.wav")
--Create a source
source = Source:Create()
source:SetSound(sound)
sound:Release()
source:SetLoopMode(true)
source:Play()
source:SetPosition(Vec3(0,0,1))
--Create a listener
local listener = Listener:Create()
listener:SetPosition(0,0,0)
while true do
--Press up/down keys to adjust the source range
if (window:KeyDown(Key.Up))then range = range + Time:GetSpeed()*0.1 end
if (window:KeyDown(Key.Down))then range =range - Time:GetSpeed()*0.1 end
if (range < 0)then range=0 end
source:SetRange(range)
if window:Closed() or window:KeyHit(Key.Escape) then return false end
Time:Update()
context:SetColor(0,0,0)
context:Clear()
context:SetColor(1,1,1)
context:SetBlendMode(Blend.Alpha)
context:DrawText("Range: "..range,2,2)
context:Sync()
end