cannot edit comment field why?

I bought a song from zune. I want to add text to the file's comment field
"bought from zune".

However, the comment field cannot be edited. In fact none of the values in
the details tab can be edited. When compared to other files that I can edit
the comment field, this song file has an additional field called "protected"
and is set to yes.

I already checked and changed the file security so that I have all the
permissions (delete, rename, ...).

How can I edit the comment field?