Shoutbox

You can use the Audiator plugin for that: http://audiator.osnetwork.sytes.net/
(make sure you read the instructions first!)

There are already many soundpacks for this plugin which you can install...

Also, besides Audiator plugin, there are several "standalone" plugins (which don't need Audiator). Look for them on the official plugin section: http://plugins.msgplus.net/ or this forum