Bukkit

You should look up how resource-packs are done in the first place before continuing here. This tut will only go in detail about custom sounds in resource packs

This will explain how the sound.json works:
http://www.accelerated-ideas.com/news/minecraft-13w42a-new-soundsjson-file--how-to-modify-and-add-new-sounds.aspx

in order to add new sounds the sound.json may look something like this:

{
  "es.join.admin": {
    "category": "master",
    "sounds": [
        "es/join/adminjoin"
    ]
  },
  "es.join.global": {
    "category": "master",
    "sounds": [
        "es/join/joinserver",
        "es/join/joinserver2"
    ]
  }
}

I strongly recommend to name sounds by the following syntax:
PLUGINSHORTCUT.CATEGORY.NAME
e.g. es.join.dosmike

the category should be master because this sound channel normaly isn't turned to 0%
Be aware: Teleporting or other events may but don't have to stop the sound.

In case you're using sound clips longer than 15 seconds i recommend you to stream then in order to prevent lag.

For all those who read this page and wanna take a look at an example you may click here. You are free to edit this pack. Note: I did not add a pack.png and you may change the package description inside the pack.mcmeta.

Some tips for using a custom resourcepack on a web server:

» Put it into server.properties (for auto dl/update) AND on some webspace in case a player initially decided to not download it
» Keep something like a syntax four soundnames like
eventsounds.saysound.hello
this just looks ways better ;)
» You may normalize the clip volume to -5 to -3 db max. otherwise it's overtuned and you won't understand a single word in e.g. ts3