I use mp3tag to change metadata for audio files. Audiobooks are usually mp3, m4a or m4b.
https://www.mp3tag.de/en/index.html
In mp3tag, there is a Comment metadata field you can fill, but not sure if that can be seen by Calibre.
My audiobook collection is small, so I don't bother using Calibre for it, sorry. Some people here do enter their audiobooks into their Calibre for library / catalogue purposes, but I think it only handles ebook files for management.