Weapon detector doesn't play the sound, when I walk through it with a tool named as a weapon. Help?

Hm, so you've given it your best so I'll help you out here. Looking at your variable names, I'm assuming you're going to want to put multiple weapon names in there, so for that you're going to need a table. To go through all of the items in this table, for this task, I'm going to be using the generic for loop (Wiki). After reading up on those, you'll find out the generic for loop is used to go through each index (value/place/item) of a table.

Once you have a for loop setup, I'm going to place it inside a isNotAllowed function to have the script check if the tool name is on the prohibited list. Assuming you're going to want to check the entire player's inventory, you'll have to loop through their backpack as well. Now onto the code: (I've commented it to help you better understand it)

local sound = game.Workspace.scanner.Sound
local prohibitedItems = {"StA-45", "Basic 9mm"} -- Assuming these are 2 separate weapons

function isProhibited(name) -- Takes the name parameter
	local is = false -- Value that will be returned
	for _,v in pairs(prohibitedItems) do
		if v:lower() == name:lower() then -- :lower() makes the string lowercase. I used it in case the capitalization varied. Remove if you wish
			is = true -- the name is a match against the prohibitedItems list. hence the if statement
		end -- Close the if statement
	end -- Close the for loop
	return is -- Return the value true/false.
end -- Close the function

script.Parent.Touched:Connect(function(hit)
	local player = game.Players:GetPlayerFromCharacter(hit.Parent) -- Method used to return a player from their charater
	if player then -- Checks to see if whatever touched it belongs to a Player's Character
		for _,item in pairs(player:WaitForChild('Backpack'):GetChildren()) do
			if isProhibited(item.Name) then -- Calls the function on the item's name
				sound:Play()
				wait(2)
				sound:Stop()
				break -- Having it end the for loop early since it already found one bad object
			end -- Close the if statement
		end -- Close the for loop
	end -- Close the first if statement
end) -- Close the connection line/function