Very few of us can carry off an absolutely believable accent for some regionalism of our own language, much less that of another. The key is: could you fool a native of Mobile, Alabama with a Southern Accent, or would someone from Liverpool, buy your variation on the multi-faceted Queen’s English? Is someone from Brisbane going to believe your Aussie accent?
[I'll not tackle the topic of believable multi-lingualism in this blog article...only accents and dialects]
Best rule of thumb? If you’re not trained, or you haven’t grown up around people speaking those dialects, then it’s probably not something that’s going to win you an audition. Interestingly, you can probably better fake a foreign-language accent (not the language itself) IF your only audience is other English-speakers.
Yet dialect coaches and training modules proliferate. These are nuances of spoken language that CAN be taught if you have that capacity, and you practice a lot. Pat Fraley is a master at this stuff…train with him if you’re interested.
If you’re in the middle of an audiobook, though, and all the sudden are presented with the challenge of delivering a line or two for a character from Dusseldorf, you might get by with a couple of the following resources:
Paul Meier is a name you’ll run across if you do research in this area. His website is: Paul Meier Dialect Services. He’ll do Skype/Phone coaching, and has this resource available: Accents and Dialects for Stage and Screen, $100, includes 12 CD’s (available at Harlan Hogan’s VoiceOver Essentials). Paul also has a website called International Dialects of English Archive, which could be valuable for, say, a Scottish brogue now that Craig Ferguson is mostly Americanized.
Jerry Blunt is another name that shows up in searches about dialect. Here’s one of his resources: Stage Dialects CD Set.
Here’s another helpful site: The Dialect Resource: The Premiere Source for Accent and Dialect Training for Actors.
And finally, this site also might be useful to you: Dialect Accent Specialists, Inc., Dialect Coaching and Instructional Recordings.