How would i add sound to this script? So when i press a button the sound activates with the script.

First off, KeyDown is deprecated so use UserInputService. Second off, just use a play function like so:

local player = game.Players.LocalPlayer
local uis = game:GetService("UserInputService") --Calls upon UserInputService
local enabled = false

local animation = Instance.new("Animation",script)
animation.AnimationId = "http://www.roblox.com/asset/?id=01311925674t"
local animation2 = Instance.new("Animation",script)
animation2.AnimationId = "http://www.roblox.com/asset/?id=903173770"
local sound = whereversoundis -- change this to where your sound is located although you can also use Instance.new

uis.InputBegan:Connect(function(obj, gp) --Function for when key is pressed
    if obj.KeyCode == Enum.KeyCode.Z and not gp then --checks if the key is z

enabled = not(enabled)

if enabled == true then --does ssj
    local UI = player.Character.Humanoid:LoadAnimation(animation)
    UI:Play()
    wait(3)
    local SSJParticle = game.ReplicatedStorage.Particles.UI:Clone()
    SSJParticle.Parent = player.Character.HumanoidRootPart
    local SSJHair = game.ReplicatedStorage.Hairs.UI:Clone()
    SSJHair.Parent = player.Character
    SSJHair.CFrame = player.Character.HumanoidRootPart.CFrame
    local Weld = Instance.new("Weld")
    Weld.Name = "UI"
    Weld.Parent = player.Character.Head
    Weld.Part0 = player.Character.Head
    Weld.Part1 = SSJHair
    Weld.C0  = CFrame.new(0, .9, 0.11) * CFrame.Angles(0,math.pi,0)
    player.Character.Humanoid.MaxHealth = 50000
    player.Character.Humanoid.Health = 50000
    player.Character.Humanoid.WalkSpeed = 80
    sound:Play() --Plays the sound

    else -- does ssj2

    local SSJ2 = player.Character.Humanoid:LoadAnimation(animation2)
    SSJ2:Play()
    player.Character.Humanoid.WalkSpeed = 16
    player.Character.Humanoid.MaxHealth = 200
    player.Character.Humanoid.Health = 200
    sound:Play() --Plays sound again. If you want a different sound for ssj2, make another sound variable
    end
    end
end)

This is merely an example so, put the sound:Play() wherever you want it to be.