http://librivox.org/
What Project Gutenberg is to e-books, LibriVox is to audio. While Gutenberg relies and volunteer hours to scan and convert public domain texts into digital documents, LibriVox relies on volunteers to read public domain texts and compile them into listenable audio formats.
LibriVox is searchable, though the search results are not as easy to muddle through as I would like. Obviously, when dealing with public domain texts, the contents of the LibriVox library will be texts that were published prior to 1923. The individual record for a title contains links to related text files, including Gutenberg and Wikipedia. The audio itself can be downloaded in its entirety as a .zip file or individual chapters. Since LibriVox relies on volunteers who read, edit, and compile the audio tracks, a book may have many different readers.
Formats include the ubiquitous MP3 and the not so common Ogg Vorbis. This latter, the .ogg file type, is playable on many devices. While most people continue to associate audio file types with MP3, that particular file type was eclipsed by superior file types, including the open source, patent free Ogg.