4. Add sound

KKE doesn't play recorded clips for impacts: it synthesizes each hit from the materials involved and how hard they met. You'll give your bodies materials, then play a sound of your own when a coin is collected.

Materials make the sound

audio.materials() returns the material table by name: Stone, Wood, Metal, Glass, Rubber, Dirt and Plastic. Give a body a material and the engine plays its impact sounds by itself, louder for harder hits, positioned in 3D and muffled behind walls.

game.lua already does this (local M = audio and audio.materials() or {}, and material = M.Wood on the crates). In throw.lua from tutorial 2, give the balls one:

local M = audio and audio.materials() or {}

at the top, and material = M.Rubber or 0, in the physics.sphere { ... }. Throw a few at the crates and at the stone floor: rubber on wood, rubber on stone. Change it to M.Metal and save to hear the difference.

A pickup chime

audio.impact(pos, material, intensity) plays an impact anywhere, without a collision. In coins.lua, after score = score + 1:

      if audio then
        audio.impact(pos, "Glass", 0.6)                         -- a light "ting"
        if score == total then audio.impact(pos, "Metal", 1.0) end -- the last one: louder
      end

React to hits

The Contact hook tells a script about every new contact between bodies:

hook.Add("Contact", "coins.hits", function(c)
  if c.speed > 6 then print(string.format("Big hit at %.1f m/s", c.speed)) end
end)

c.a and c.b are the two body ids, c.pos where they met, c.speed how fast they met, and c.materialA / c.materialB their materials. Use it for damage, particles, or a score for knocking things over.

Players who can't hear

The audio settings include a sound visualizer, which shows on screen where sounds come from, for deaf and hard-of-hearing players. Your materials feed it too, so a game that uses them is more accessible without extra work. See Audio.

What you have

A small game: a character to move, a level, things to throw and break, coins with a score, and sound. From here, the Lua API reference and Scripting in Lua cover everything else scripts can do.