Actually, they do count, but you have to (as far as I know) install Quicktime in order to listen to them.
About the Avi question, the same weird thing happened to me, when I did a fresh rescan for the files on the disks on my new computer. Some avi files got recognized as audio files. But at that point, no xvid, divx were installed. After I installed the codecs, the files still under audio but can be played, with MC or other Players.
But in my opinion the problem lies within the files not in MC, because the majority got recognized correctly as videos.
