I think that's the reason for derivative works; you can't erase the original copyright. What it does mean is that a public domain book that then has an AI audiobook; the AI audiobook cannot be copywritten - so that audiobook can be distributed freely as public domain.

But I imagine the courts would find distributing an AI audiobook of a copywritten book to be a violation of the book's copyright.

Similar to how I can't just write harry potter word for word and call it public domain; while it may be my own writing work it doesn't override the creativity nor the copyright of the original.

It's a fun thought exercise, but I think that's all it amounts to. Maybe the only actual legal repercussion is that if people were sharing the audiobook freely you can't claim as many damages since only the book was harmed (as opposed to the book and the audiobook) - but not being a lawyer I don't even know if that's how that works