If you are trying to work on dialects for this show, there are a few very good sources.
You can listen to native speakers at the International Dialects of English Archive: http://web.ku.edu/idea/
If you're looking for something that actually teaches accents, check out http://www.accenthelp.com
--There you'll find downloads that teach you how to speak with accents and there are recordings of natives included.
You can also order dialect CDs from http://www.dialectresource.com
which will include instruction and examples.
(Full disclosure: I'm a dialect coach and contributor to both IDEA and AccentHelp.)
Usually it is very hard to audition properly for a show that requires a dialect without actually doing the dialect. Being able to do the accent will always put you a step above anyone who is not able to pull it off. These sources should serve you well for both auditions and in production.