Thanks for your reply, and sorry for my delay - I have been troubleshooting when I have minutes here and there.
I am using a Mac with Big Sur. It's audiobook files from Harry Potter books that I am noticing aren't transferring properly. I added them years ago by CD and haven't had issues with the files until the past year or so. At some point, I uploaded them to Google Drive so I could transfer them between computers as needed (maybe that threw things off?). When I bought a new MacBook, I had my music stored on iCloud, and I thought perhaps that was it. I transferred my files over to my Home folder, but that doesn't seem to have done the trick. I checked on the files that aren't transferring, and they are all showing as coming from the same folder. Here are some screenshots. What's even more confusing is that I have tried accessing the files both through Books and through Music, and they don't show up properly in either app - but they show up incorrectly in different ways.
Thank you for trying to help!
Img 1: screenshot from first chapter files in Music on Macbook
Img 2: screenshot from first chapter files playlist in Books on MacBook
Img 2: screenshot from first chapter files in Finder
Img 3: screenshot from first chapter files in Books on iPhone
Img 4: screenshot from first chapter files in Music on iPhone