Not sure if this will help but a friend from another cyber haunt recommened the
VLC player to someone for something or other. (May have been for MP4s.) It's free and seems to work rather well. Not sure if it works with AAC files though as I haven't tried it with those yet. This little program has more stuff in there than I have any clue of what they are talking about!
-Edit-
Apparently, it does handle AAC. Here's the
Full Features List. Like I said, it's a nice little piece of free software.
But, it's not a converter. Hmmmm, keep searching....