The Drama Library collects tapes and CDs for actors and directors to use during rehearsals or productions. The most detailed material includes a discussion of the phonetic alphabet, vowel sounds, and word stress. Many dialect tapes also include information on breathing, use of the tongue, and positioning of the jaw. Most resources provide frequent opportunity to practice as you listen.
Plays with Dialect:
Have a particular dialect in mind, and looking for a play that features that dialect? A good source is Play Index located in Drama Reference Z5781.P53. In the online version type the dialect you're looking for in the search box, for example, "dialect Scottish" or "dialect African American".