Adding Music Stop, and Music Play?

You aren't calling those methods, you're only sort of indexing them. If you looked at the output, you would see it cursing you.

:Play should be :Play() and :Stop should be :Stop()

1waffle is correct. If you are too lazy to add the parenthesis to your code, here you do :P

function CountDown()
    wait(5)
    for i,v in pairs(game.Players:GetChildren()) do
        coroutine.resume(coroutine.create(function()
            local Time = 30
            local Hint = GuiModule.IntermissionHint(v,"There will now be a brief intermission..")
            wait(1)
            Script.Parent.Music:Play()
            while Time > 0 do
                Time = Time - 1
                Hint.TextLabel.Text = "Intermission: "..Time
                wait(1)
            end
            Hint.TextLabel.Text = "Starting game.."
            Script.Parent.Music:Stop()
            GuiModule.RetractIntermissionHint(Hint)
        end))
    end
    wait(30)
end