Bukkit

This mod requires spout and will only work for spoutcraft clients and a filehost for the sound files. I personally use dropbox for that but they are a lot of other options out there.

Installing Spout and Spoutcraft

Spout is an server plugin that allows the server to work closer to the client. With spout the server can show a custom GUI window for the client or in this case, play custom sound effects. But most of the new features if not all requires the user to use Spoutcraft. Spoutcraft is a modified Minecraft client that unlocks all the features of the Spout servermod. A player using a normal Minecraft client on a spouts server will still be able to play but many of the new features wont work for them. This mod will not do anything for players without Spoutcraft. You can read more about them here and here

Installing the mod

Make sure you got Spout installed first then drag the jar in the server plugin folder. Upon the first run it will grenerate the default config file looking something like this

sound:
filePath: http://your.site.here
settings:
range: 45
volume: 70
nodeseparation: 20
radius: 5

Upload the sound pack unzipped to you filehost and then point http://your.site.here to the folder where the files are located, an FTP server or Dropbox are recommended for this task. For example say that you file is hosted at www.host.com/minecraftstuff/sound.ogg then filePath should read www.host.com/minecraftstuff/

At the time of writhing if you want to have custom sounds for a biome it will have to have the same name as the file you want it to replace from the default soundpack.Here is a key over what they are.

The simplest way making a sing play a sound is

[BiomeSound]
mysound.ogg

This sign will now attempt to play filePath/mysound.ogg. Here you don't have to use ogg vorbis, wav and midi work as well. Just remember that you have to add the file ending yourself.

Most users don't need to change these settings but here comes a brief description of them. Range is how far the sound is possible to hear. 45 is a good setting and unless you got something modifying the move speed of you players it should be fine. Volume is the volume the sounds play at. You didn't expect that did you? Node separation forces players to move before the mod will scan the environment for new sounds to play. A low value is more demanding on the server and client as more sound will play. Radius is how far the the plugin will look for lava and signs, it will have a pretty big drain on the servers so don't put it to high. If you put it at 16 it will scan pretty much a whole chunk.

If you want to use you own sounds keep in mind that most of them need to be about 10-15min long as spout/minecraft got no way to loop them.

Configuration

Every setting is configuarble using permission, everything should work without any permission but only op can do somethings.