Calibre's main function is ebooks, but MP3s and audio books can be added to calibre. Drag and drop them to the book list and they will be added. It is a little ugly (see attached) but it works. This might be a good case for having a second calibre library.
Be sure to add the
[Metadata Reader] AudioBooks plugin prior to adding the mp3s that way you have a chance of extracting the id3 metadata from the mp3 upon adding. You also might want to read the entire [Metadata Reader] AudioBooks plugin thread prior to attempting this.
Good Luck.