It wouldn't be to save the cheap coat of a voice actor.
It's so they can play the audio to their AI for free without having to say it was fed a copywritten text. It would also get better at telling stories, depending on the quality it was fed.
But the main advantage is training it to follow a long verbal narrative. And decide if it's better to transcribe it for full reference, or just make a summary as the story goes and risk missing an important bit.
Then to repeat it in the AI's "own words". This would make a huge loophole for exploiting famous authors. If you feed AI the text, the author can argue it was trained on it. If the AI just listened to it and makes a summary and remembers the structure. Derivative works of famous authors can be claimed to be no different than a human emulating popular authors that they had read.
They're just trying to find a way around using the full text, and reading it aloud might be enough.