I'm a bit late to your question, but I actually was coming here on a very similar topic. For audiobooks, if the print is written in dialect, it should be spoken in dialect. But for print books, I think you need to be careful writing dialect. There's a huge difference in the dialects used in Huck Finn and the ones in The Help.
But, I just finished My Sister, The Serial Killer, and it is set in Nigeria. They hired an American actress for the reading, which I don't have a problem with, but...she voiced the protagonist and her sister with American accents! Everyone else had a Nigerian di