How to crop a sound?

So I made this script:

function onPlayerAdded()
    local RandNum = Instance.new("NumberValue")
    RandNum.Value = math.random(10)
    for i,v in pairs(game.Players:GetChildren()) do
        if v:IsA("Player") then
            RandNum.Parent = v
            RandNum.Name = "RandomNumber"
                if RandNum.Value <= 3 then
                    game.Workspace.Sounds.Firework:Play()
                    local LuckMessage = Instance.new("Message", game.Workspace)
                        LuckMessage.Text = RandNum.Parent.Name .. " got lucky! Good job!"
                        wait(5)
                        LuckMessage:Destroy()
                elseif RandNum.Value >= 8 then
                    local UnLuckMessage = Instance.new("Message", game.Workspace)
                        game.Workspace.Sounds.Suspense:Play()
                        UnLuckMessage.Text = RandNum.Parent.Name .. " got unlucky. Sorry."
                        wait(5)
                        UnLuckMessage:Destroy()
                            local UnLuckExplode = Instance.new("Explosion")
                                for i,v in pairs(game.Workspace:GetChildren()) do
                                    if v:IsA("Model") then
                                        UnLuckExplode.Parent = v.Torso
                                end
                        end
                end
        end
    end
end

game.Players.PlayerAdded:connect(onPlayerAdded)

for _,v in pairs(game.Players:GetChildren()) do
    if v:IsA("Player") then
        onPlayerAdded()
    end
end

And the Firework sound effect just goes on too long. If I try to put a wait(18) there, then it will delay the message. Can someone help me? Thanks! :)