Mumble

Mumble is an open source application used for real time voice communications, usually for gaming. It is optimised for low latency voice transmissions and has exceptional quality even at low bitrates. This makes it well suited for live broadcasts of conferences, interviews and other situations where speech intelligibility is important. You can read more about mumble here on Wikipedia.

Mumble for Broadcasts
This only covers configuration for listening into broadcasts. If you wish to use Mumble otherwise, many general setup tutorials are available on the Internet which will guide you through the process of calibrating and configuring your input devices.

Downloading Mumble
Mumble is available free of charge from Sourceforge at http://mumble.sourceforge.net/. As of the time of writing, there are builds available for Windows, Mac OS X, Linux and iOS. Most Linux distributions have mumble in their official repositories.

Installation
The screenshots shown here are for a fresh installation on Windows 7, though the interface should be similar for Mac OS X. Run the executable after download (Windows) or download and open the disk image then drag and drop the Mumble application to your Applications folder (Mac) and follow the prompts.

By default, only the client is installed. You can uncheck Mumble (backward compatible) as it is not required in most circumstances. Murmur is not needed unless you wish to set up your own server.

Configuration
After installation has finished, open Mumble. If this is the first time running Mumble, the Audio Tuning Wizard will open.

Click cancel to close the window, and cancel again to dismiss the Certificate Management wizard.

You should now be presented with a window to choose which server to connect to. (If this does not automatically appear, go to Server > Connect). Click on Add New and enter the address of the server you wish to connect to. The port can usually be left on the default setting (64738). Pick a user name and assign a label to the server to help you identify it. Click OK to save the settings.

The server you added should now appear under the 'Favorite' list. Highlight it and click connect to join the audio stream.

Your window should look something like this if you have connected successfully. Be sure to click the microphone icon (between information and headphones) to mute yourself. Listeners in the stream appear in the right hand panel of the screen. You can send and view messages to others in the left hand pane similar to traditional instant messaging.