I’m not a lawyer, but I have voiced an attorney in multiple audiobooks. Just because an author has passed away or isn’t easily discoverable, or the book is out of print, doesn’t mean you’re free to create the audiobook of her book!
Before you can record an audiobook, you must do due diligence to determine whether the book is in the public domain or still under copyright. When a work is still under copyright, the rights holder is the person or company that owns the audio rights to the book. The rights holder (RH) will receive the royalties from the sale of the audiobook.
If the book is still copyrighted, the RH could retain the audio rights and hire you as an independent contractor to produce the audiobook. You also could license the audio rights and become the rights holder. If you’re interested in this second option, I highly recommend you purchase my webinar with attorney and audiobook producer/director/distributor Jessica Kaye on this topic using the link on my Shop page.
The links on this page will help you understand the copyright laws in the US and UK and do your research to find the rights holder.