This script only works sometimes?

So I have this script put into bricks so that when they are clicked, a sound will play. The problem is, it only works sometimes, at random intervals.

-- This script goes into a ClickDetector within a part.

local function onMouseClick(Player)
    if not Player.PlayerGui:findFirstChild("ClickAudio") then 
        local ls = Instance.new("Sound")
        local length = 1 -- How long the sound plays before removed.

        ls.SoundId = "http://www.roblox.com/asset/?id=" -- Audio id here.
        ls.Name = "ClickAudio"
        ls.Volume = 0.5
        ls.Pitch = 1
        ls.Parent = Player.PlayerGui
        ls:Play()

        game:GetService("Debris"):AddItem(ls, length)
    end
end
script.Parent.MouseClick:connect(onMouseClick)

The audio will play, but not all of the time. And I can't just spam the button a bunch of times, because it won't play. I have to wait a few minutes and try again, and it still might not work.