I found a way to play Librivox free books!!
When an *.mp3 file is in the Audible subdirectory, the Kindle automatically treats it as an audio book!! :-D
So you download all the mp3 chapter files from a Librivox book,
you concatenate them (I us "cat" under Linux or with Cygwin),
then you edit the ID3 tag info (otherwise it shows the ID3 tag info for just the last chapter).
All mp3 files under Audible (even in subdirectories) show up as one book each in the home list.
(Ogg vorbis files don't get recognized. :-P )
Does anyone know how to turn a list of mp3 files into an audiobook with *chapters* which the Kindle audiobook reader will recognize?