Through a quirk in our copyright law, around 80% of American books published before 1964 are not copyrighted and are therefore in the public domain. The LibriVox project takes advantage of this by using volunteers to record chapters of books outside of copyright and release free public domain audiobooks!
They're been recording free audiobooks since 2010 and currently have over 16,000 catalogued works, with 93 new books added within the last month alone. You can search by author, title, language, or genre / subject. You don't even have to make an account in order to access the free audiobooks. Simply click on any book, then download the whole book's worth of audiofiles from the sidebar. You can also listen to any chapter right in your computer or phone's browser without any extra software required.