Simple pong game in lua with LÖVE

pong.lua

ball = {}
ball.x = 300
ball.y = 200
ball.vel = {}
ball.vel.x = 3
ball.vel.y = 1
ball.height = 30
ball.width = 30

map = {}
map.offset = 20
map.width = 600
map.height = 400

a = {}
a.width = 10
a.height = 80
a.y = 200
a.x = 580

b = {}
b.width = 10
b.height = 80
b.y = 200
b.x = 40

a.score = 0
b.score = 0

sound = love.audio.newSource("pong.wav")

scoreFont = love.graphics.newFont(30)

function love.update()
	ball.x = ball.x + ball.vel.x
	ball.y = ball.y + ball.vel.y

	--map boundaries
	if ball.x >= (map.width + map.offset) - ball.width then
		b.score = b.score + 1
		ball:reset()
	end
	if ball.x <= map.offset then
		a.score = a.score + 1
		ball:reset()
	end

	--walls bounce
	if ball.y <= map.offset then
		ball:bounce(1, -1)
	end
	if ball.y >= (map.height + map.offset) - ball.height then
		ball:bounce(1, -1)
	end

	--paddles bounces
	if ball.x > a.x - ball.width and ball.y <= a.y + a.height and ball.y >= a.y - ball.height then
		ball:bounce(-1, 1)
		ball.x = ball.x - 10
	end 
	if ball.x < b.x + 5 and ball.y <= b.y + b.height and ball.y >= b.y - ball.height then
		ball:bounce(-1, 1)
		ball.x = ball.x + 10
	end

	--keys testing
    if love.keyboard.isDown("up") and a.y > map.offset then
		a.y = a.y - 2
    end
    if love.keyboard.isDown("down") and a.y + a.height < map.height + map.offset then
		a.y = a.y + 2
    end
    if love.keyboard.isDown("w") and b.y > map.offset then
		b.y = b.y - 2
    end
    if love.keyboard.isDown("s") and b.y + b.height < map.height + map.offset then
		b.y = b.y + 2
    end
end

function ball:bounce(x, y)
		self.vel.x = x * self.vel.x
		self.vel.y = y * self.vel.y
end

function ball:reset()
	ball.x = 300
	ball.y = 200
	ball.vel = {}
	ball.vel.x = 3
	ball.vel.y = 1
	ball.height = 30
	ball.width = 30
end

function love.draw()
	love.graphics.rectangle("fill", ball.x, ball.y, ball.width, ball.height)
	love.graphics.rectangle("line", map.offset, map.offset, map.width, map.height)

	--draw paddles
	love.graphics.rectangle("fill", a.x, a.y, a.width, a.height)
	love.graphics.rectangle("fill", b.x, b.y, b.width, b.height)

	love.graphics.setFont(scoreFont)
	--draw score
	love.graphics.print(b.score .. " - " .. a.score, 280, 40)
end

 love .
Error: Could not open file pong.wav. Does not exist.
stack traceback:
[C]: at 0x7f8418e76160
[C]: in function 'newSource'
main.lua:30: in main chunk
[C]: in function 'require'
[string "boot.lua"]:429: in function <[string "boot.lua"]:275>
[C]: in function 'xpcall'

If you don't have a sound file, delete this line.
sound = love.audio.newSource("pong.wav", "static")
If you do use this instead and make sure you have a pong.wav file.