A script of mine works in solo, but not online?

Earlier today a friend of mine requested a few scripts, and I went ahead and made them because I've only started cracking down on learning Lua earlier this year and really needed the practice.

One of these scripts was pretty simple - make a sound play when a part is touched. After a while I had whipped this up.

local playing = false

function MakeSound()
    if playing == false then
        local sound = Instance.new("Sound")
        sound.Parent = script.Parent
        sound.SoundId = "http://www.roblox.com/asset/?id= " --Sound ID goes here
        sound:Play()
        playing = true
        wait(1)
        playing = false
        sound:Destroy() --can't leave all these played sounds laying around
    elseif playing == true then return
    end
end

script.Parent.Touched:connect(MakeSound)

For some reason it worked in solo, but not online. I realized I over-complicated it so I changed it to simply play a pre-existing sound when touched, and now it works online.

But I'm curious just in case this problem ever arises again in my scripts... what IS the problem? Why'd it work in solo but not online?