How do I make a step on part plays sound once script?

So lets say someone steps on a part. Then they hear a sound, but when they try to step on it again nothing happens. And when the sound played the first time nobody but the player who stepped on it hears it.

You'll have to play the sound locally on the client of the person who stepped on it.

Simply stated, you should use a touched event, then play the sound on the client using some remote event or something.

Something like this might work:

Be sure to insert the RemoteEvent object into replicated storage

--Server Script

local ReplicatedStorage = game:GetService("ReplicatedStorage")
local Players = game:GetService("Players")
local RemoteEvent = ReplicatedStorage:WaitForChild('RemoteEvent")

Part.Touched:Connect(function(hit)
	if hit.Parent:FindFirstChild("Humanoid") then
		local character = hit.Parent
		--get the player
		local player = Players:GetPlayerFromCharacter(character)
		--fire the event on the client
		RemoteEvent:FireClient(player)
	end
end)

--local script
--this should be parented to the player 

local ReplicatedStorage = game:GetService("ReplicatedStorage")
local RemoteEvent = ReplicatedStorage:WaitForChild('RemoteEvent")

local Part = workspace:WaitForChild("Part")
local Sound = Part:WaitForChild("Sound")

RemoteEvent.OnClientEvent:Connect(function()
	if Sound then
		Sound:Play()
		--wait for the sound to stop playing
		Sound.Ended:Wait()
		--destroy it so that it cant be used again on this client
		Sound:Destroy()
	end
end)

If you only want the player who stepped on it to hear, I'd suggest using a Local Script, let's get to the code:

local hasStepped = false
part.Touched:Connect(hit)
	if hasStepped == true then
		return
	end
	local humanoid = hit.Parent.FindFirstChild("Humanoid")
	if humanoid then
		sound:Play()
		hasStepped = true
	end
end)

Now let's break the code down:

local hasStepped = false

First of all we declare a variable called hasStepped, to store whether the player has or hasn't stepped on the part yet.

part.Touched:Connect(hit)

In this section, we tell the part to connect to a function when it gets touched using "hit" as an argument. If you wonder what "hit" is, try doing:

part.Touched:Connect(function(hit)
    print(hit)
end)

It will print out a part of the player's character, and every body part is the child of the character itself.

if hasStepped == true then
	return
end

Now we check if the hasStepped variable is true, if it is, it's going to skip the block of code, not playing the audio.

local humanoid = hit.Parent.FindFirstChild("Humanoid")

Declaring a variable for the humanoid so we can check if it's true on the next line of code.

if humanoid then
	sound:Play()
	hasStepped = true
end

Now if it is a humanoid, we play the sound and set hasStepped to true, meaning that the function will never run again.

You can also use:

script:Destroy()

But instead of using a variable, it destroys the script and you're never able to use it again.