One of my Listservs announced that the Library of Congress has classified some of its audio recordings from around the country into a set of American English Dialect Recordings
organized by place (click “C” for Canada). The core is probably the set of collections made by linguist Walt Wolfram, but other samples are included, and the collection also includes some notable figures such as Franklin and Eleanor Roosevelt and Amelia Earhart. These last two recordings are old enough that you can see how speech in formal settings has changed over time.
The tapes are primarily open-ended conversations or speeches, but the extensive metadata gives you a good context of who, what and when. Many of these were recorded on site, so audio quality for spectrographic analysis is probably hit or miss, but it does have some good samples, and they are available in the .wav format as well as MP3 and Real Player. Note also that samples were recorded across a period of several decades (from the 30s to the 80s), so dialects in that area may have changed since the original recording.
However, they are freely available for educational or research use, so that’s a major benefit. This collection was organized by the Center for Applied Linguistics (CAL), so I am happy to see this as a use of our tax dollars at work.