Librivox boasts an impressive library of 5,000 audibooks read by volunteers from all around the world. All books are completely free and can be listened to on a large range of devices. The search feature supports searching by author, title or reader.
Open Culture offers thousands of resources that are free of copyright, and can be used by anyone. Their list of classic books informs users where the free resources are to be found, and in the case of the audio books, it specifies if the file can be downloaded from Audible (free app for iphone and android), iTunes, Spotify and other MP3 options.
This is a great site for high school English students to peruse should they need or want audio support for their novel study portions of their classes.