Both the Peninsula Library System http://plsinfo.org/ and the San Francisco Library System http://www.sfpl.org/news/overdrive.htm have audio material available online now.
I am not sure about the Peninsula library system, but audio material from the SF system is free for library patrons and anyone can get a library card.
You might want to consider using the SMART Board or time in the computer lab to show your students how to access this material online, how to use it, etc.
The audio material from the SF system can be downloaded onto a computer, burned onto a CD, or downloaded onto an MP3 player. All sorts of material are available - eBooks, audio books, music and video downloads. Audio books include fiction, non-fiction, adult, young adult, children's, etc.
And the best part? No late fees!