Using Banshee 2.6.2 with LinuxMint 17.1 I could import my music and my videos succesfully. When I import an audiobook (mp3) Banshee seems to import the file but does not display anything in the category "Audiobooks".
Have you checked your Music collection? I don't know whether there's
anything better than Banshee could do in some cases (and whether it is
Banshee or the underlying libraries that they use in others) but I've
had audiobooks and videos both turn up in Music. Dragging and dropping
them to the right section seemed to work for me.