I need help on camera manipulation, have any idea?

The best way to do this is check the distance between the CurrentCamera and the Head. I would use magnitude for this like so

local Magnitude = (CurrentCamera.CoordinateFrame.p - Head.CFrame.p).magnitude

Usually the magnitude is around 1 stud when you are zoomed into first person.

if Magnitude <= 1 then
-- do stuff()
end

To make this work though, you would have to put this in a while loop because I'm pretty sure .Changed doesn't fire for CoordinateFrame. Also, to make your sound only run when you get put in first person, we can make a variable named ZoomedIn and if it is already true than not play the sound.

local ZoomedIn = false

while true do
	wait(.01)
	local Magnitude = (CurrentCamera.CoordinateFrame.p - Head.CFrame.p).magnitude
	if Magnitude <= 1 and ZoomedIn == false then
		-- sound stuff
		ZoomedIn = true
	else
		ZoomedIn = false
	end
end