help with music gui's? [Half Solved]

You forgot the parenthesis around 'Play'.

local button = script.Parent
local sound = button:WaitForChild('Sound')

sound:Play()

local function onButtonClick()
    if sound.IsPlaying then
        sound:Stop()
    else
        sound:Play()

    end

end


button.MouseButton1Click:connect(onButtonClick)

I think one of your problems is where the sound is located. To make the player who pressed the button hear the sound and no one else, you would insert the sound into the player. You should be using a local script as well. You would then get the player by using,game.Players.LocalPlayer.Sound:Play(). Also you don't need thesound:Play() on line 4. Since you're going to be adding a lot of sounds to your music GUI you might want to insert a sound folder. Just make a folder in workspace and fill it up with the sounds you want. Then when a player joins put the folder into the player,

game.Players.PlayerAdded:connect(function(plr)--Returns player
   local SoundFolderClone = game.Workspace.SoundFolder:Clone() --Clones folder
   SoundFolderClone.Parent = plr --Puts folder in player
end

If my memory serves me right this should work. If not use :GetChildren() from the folder before cloning it. I hope this helped! Good luck!