I tried various programs to mass identify mp3's with sketchy success. If the mp3's were ripped directly from the retail CD it should have the id3 data already and LMCE will simply download appropriate art/info for it. If that is not happening, I've found the best way to do it is to sort your music into folders then use the webadmin to assign attributes. LMCE doesn't utilize folders as a system of sorting, they're only to make it easier for you to identify songs from the same artist/album.
With avi files, bite the bullet, head into webadmin and wear out the search amazon button. Believe me, it's the best way.