If it’s setup how I think then I may know an easy fix. If it isn’t then it could potentially make a bit of a mess that’ll take work to correct. Here’s what I need to know:

Check the file structure in Windows Explorer. It should be something like

iTunes -iTunes media —Artists —-Albums

Before doing anything you want to make sure that all your m4a files are under iTunes Media. Doesn’t matter how they’re organized within that. Depending on your setup it may not have those sub folders. That’s fine.

You just want to be sure that the files aren’t elsewhere on your computer before we do anything. That could result in duplicate files that will eat up storage space and be a nuisance to clean up.