You should look to use a third party's chat app because there are many challenges involved with doing a multiuser application.
You need both a client component (in java or flash) and a server component (in java, flash, php, asp, c++, c or anything you want).
Building a server component with Java is a very advanced task because you'll need multithreading, shared memory, socket management, etc. Building the client component with Java is also advanced because you'll need to spend some time learning about the java audio libraries, use sockets and sign the applet.
Building the server with PHP or ASP is simpler because you don't need to explicitely use sockets and multithreading (the web server takes care of that). But, you'll still need to use shared memory and thread synchronization.
The easiest approach is to use Flash at the client and at the server use flash again running under the Flash Media Server or the old Flash Communication server. This is easier to code but a lot harder to host because most web hosts don't offer Flash Server hosting and those that do are really expensive. You'll either have to setup your own server or buy a dedicated server which is even more expensive than a flash server web host.
For setting up your own server you need to be familiar with how to setup an iis or apache environment, setup the DNS for your domain, setup the Flash Media server in case you decide to go with Flash, and get a new box. You'll also want a business package from your ISP because you want a static IP, open ports (my ISP blocks port 80 for example) and a fair amount of bandwidth (since you're looking to do a voice chat).
Notice that if you go with Java, C++, C at the server as well then you have the same hosting issues as if you went with the Flash Media Server because most hosts won't enable you to load up a custom built server listening on some port, unless you have a dedicated server.
And FYI, even if you go with a pre-built chat app, unless the company you buy it from also hosts it, you'll also have issues hosting its server component, since it's probably not built with ASP or PHP.