Many people asked for a way to:
1: manage audio books without copying them into the Calibre DB
2: manage M3U (playlist) files
So, I've added feature request #2, which gives you a workaround for #1.
See version 1.0.77 and following.
* : by hand, or with a tool (like mp3tag) make an .m3u playlist file of your .m3u files
* : edit the .m3u file in notepad (or notepad++ or vi or ...) a simple text editor
* : ensure the file references are ABSOLUTE PATHS
the .m3u file will be copied into calibre, so relative paths get broken.
* : in calibre, add books, set for file type all (*.*)
locate your .m3u file and add it
CAVEAT: if your .mp3 file is online ( http://some.site/somefile.mp3
), a local copy if retrieved for parsing of metadata. If that happens to be a stream, it will never finish the copy, but will fill your local disk drive. Don't do streams!