Vox Machina, to put it simply, is a speech synthesizer program for Mac OS X. It utilizes Apple's Speech Synthesizer technology, which enables it to speak text out loud and save spoken text as audio files (either uncompressed AIFF or MP3-encoded audio). Vox Machina also sports a lip-syncing feature where the relevant lip and mouth movements corresponding to spoken phonemes are displayed with a series of images.
Vox Machina is primarily intended for entertainment, but it can be useful for creating spoken to audio to listen to later (e.g. on an iPod).
Vox Machina is free, open-source software distributed under the terms and conditions of the Free Software Foundation's GNU General Public License, which should have accompanied your copy of the program. That being said, if you enjoy Vox Machina and would like to see further development, you can make a donation. They are highly appreciated.
Using Vox Machina should be fairly easy. Just type in the text you want to be spoken/saved to audio file, and press either the "Speak" or "Save Audio" button, depending on which function you want. MP3 compression is done using the open-source LAME MP3 encoder, which can be configured to some extent by pressing the "Encoding options" button.
The author of Vox Machina is Sveinbjorn Thordarson. Contact him if you encounter any bugs or problems.