Historically, being told that a voice recording is AI generated would be enough to tell you to expect basic TTS robotic voice, but with advances in AI voice generation we're approaching the point where AI can sound as good as real humans - it's not yet to the point where it's easy to generate an audiobook as good as a professional reader, but that point will come in the not too distant future.
And equally on the other side, something being recorded by a human doesn't automatically mean it has the quality of a professionally-read audiobook. This is something LibriVox has always had to deal with, by gatekeeping which volunteer recordings to either give feedback requesting improvements to or to not use at all.
In some but not all cases, an amateur human reader can already be as good as a professional, that will soon be true for AI. For both AI and humans it will remain the case that some efforts are not as good, but the line between them (for quality) isn't going to be whether or not they are AI - though I do agree that AI or not should also be labelled.