Addons for the game are created, most simply, by making a new folder in the Interface/AddOns
directory in your game folder and populating this with the core files for your addon. These files should include a "Table Of Contents" file which contains information about your addon, and script(s) which are created using the Lua scripting language (with some custom WoW functions and tables and other bits). To properly get started with this, Wowpedia is generally a pretty good guide, and I also recommend this tutorial.
In your specific situation you should just be able to listen for a game event and then do your custom stuff (i.e. playing a sound) in the desired situation. There isn't actually an event for killing blows at the time of writing, however if you register the COMBAT_LOG_EVENT_UNFILTERED
event and look for the PARTY_KILL
combat event, calling playSoundFile
if the sourceName (arg4) matches the player's name (UnitName("Player")
), you should be set.