They're very popular, but no where near as bandwidth heavy as videos
Itch is not a media content hosting site, like youtube. You do not host videos on Itch. You might sell your indie movie here. But you do not upload your vlog here. Or a podcast.
An audiobook is not a primary media. One does not create the story in audio format. It's literally someone reading a book. An existing book.
Itch has 20000 items in the book category. I do not think, we need a category for < 100 items.
https://itch.io/books/tag-audiobook
https://itch.io/books/tag-audio
But one might want to suggest to have a "suggested" tag for these. "suggested" tags are the tags that are in the list and will appear in auto-complete when typing a tag in the tag selector boxes.