Live Audio Webcast from Linux Server

I want to develop a "Webcast" type of Web site that will include a live audio stream.  The application is that we are going to have a presenter present a topic that will include some graphics on the screen.  The end user is then listening to a broadcast and watching various grapics display on the screen.  

We will be using a Linux RedHat 5 server and the website code will be written in php 5.  

The source for the audio will be a phone call that will be encoded and sent to the server from a local computer.  

My question is what is the best way to broadcast live audio from the server.  What software needs to be on both the server and on the local machine?  

I've searched thru EE but most of what I'm seeing is technology that is a number of years old or is a paid service.  I'm interested in buying the software needed to host this totally on our server.

Thanks.


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