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.