Books Should Be Free is an outstanding resource that contains hundreds of audio books totally free to download as an mp3 or iPod audio file. Each audio book is in the public domain, meaning nobody owns the copyright any longer and anyone can distribute them. There are many many classic books that can be shared with your struggling readers, ESL Students, or someone wanting to simply listen to a great story.
If your classroom is readying a classic literary piece, you will probably be able to find the audio book on Books Should Be Free. This is a great tool that should be in any teacher's "box of tricks."