Of course, I am fairly ignorant about American regional accents, but I could tell that in the reading of Stephen King's novel the people of Maine spoke like this and the people of Dallas and Fort Worth spoke like that. The Jewish pawn broker was given a voice like Burt Lancaster, which seems authentic and elegant. I took the various accents of Eutopia on trust - Chicago, black east-coast intellectual, Montana etc. - but I am sure the voice artist knew what he was doing. Jacobi, as you might imagine, was truly excellent. Holmes stories rapidly introduce cameo characters, defined by age, class and background. Jacobi adjusted his voice delivery to each case: very enjoyable.
Now I am listening to a new book, which promises to be right up my street. It's called Bitter Seeds and it is fantasy/horror novel set during the 2nd world war with the Nazi's deploying X-Man style ubermenschen and plucky British pagans invoking the ancient powers (as I think indeed happened, though to what effect I could not say). It's set in Europe, but written by an American author, and read by an American voice artist.
I think the voice artist is very good, in general terms, but he does not understand British accents. It's jarring to hear a butler from Nottinghamshire speaking in a cheeky Cockney accent. The Polish spy had a Russian accent, and so on. You don't even know how much you know about voice, until you hear it done wrong. This is not me saying that American readers are no good, far from it, I have enjoyed so many lately. It could easily happen the other way if a British reader assayed an American book, giving every character the same drawl.
I was talking to H about this and he thinks that there are more rigidly defined regional accents in Britain than other countries. I argued that possibly we are simply more aware of the variation in our own country, and it may be similarly complex elsewhere, but we don't know it. I honestly don't know whether (say) a German or a Malaysian can distinguish the town that someone comes from, and their relative prosperity, from their voice. I would be interested to hear other people's opinions about that.