Music isn't playing when i touch a part??

You can't call LocalPlayer from a server script. If you wanted to achieve this, you could send a RemoteEvent to the player. The RemoteEvent would then allow the sound to be played in the local script.

It works in studio because there is only one local player (hence game.Players.LocalPlayer) being an instance.

The following script will work for your instance with a little revamp:

-- SERVER SCRIPT (PUT THIS IN YOUR PART)

local event = game.ReplicatedStorage.MusicEvent -- This is the remote event.

script.Parent.Touched:connect(function(hit)
    local player = game.Players:GetPlayerFromCharacter(hit.Parent)
    if player ~= nil then
        event:FireClient(player)
    end
end)

-- LOCAL SCRIPT (PUT THIS IN STARTERGUI)

local event = game.ReplicatedStorage.MusicEvent -- The remote event.

local debounce = false
local cooldown = 50

game.ReplicatedStorage.MusicEvent.OnClientEvent:connect(function(info)
    if debounce then return end -- If the debounce is true then stop the script
    local sound50 = Instance.new("Sound", game.Players.LocalPlayer.PlayerGui)
    sound50.SoundId = "rbxassetid://184219038"
    sound50:Play() -- More efficient way to play.
    wait(40)
    sound50:Destroy() -- More updated version of removing an object
    wait(cooldown)
    debounce = false
end)