Actually, there are a number of sites that produce 'free' audio books (essentially just a MP3) that, as
Don't Panic! notes above, is more like a tape or CD recording rather than a text to audio reader.
Like -
http://www.audiobooksforfree.com/screen_main.asp
Then any MP3 player on your PDA (Windows Media, WinAmp, etc) will be able to do playback.
Oh, and
Don't Panic! - 42.