You can also search for "accent tags" on Youtube. There is a specific list of words to read and questions to answer and people from around the country (and world) have recorded themselves to provide examples of their local speech.
Encyclopedias are for answering questions that have known, generally agreed upon, answers--the fact-based "who," "what," "when," and "where" questions. For "why" and (non-procedural) "how" questions, you usually need to do research in books and journals.
We often think of sources of background information as the kind of neutral "we all agree on these facts" sort of article that you can find in an encyclopedia or textbook, and that is one type of background source. However, providing background is a function, not a type or format of information source. Primary documents, whether textual, visual, audio, or material, can also provide foundational information if their authenticity is accepted. If you are analyzing a particular speech performance, you can read an encyclopedia article or textbook about the characteristic dialect or speech habits of the relevant/reference population, but you can also listen to a half dozen examples of people speaking who belong to that group for comparison or examine a map of American dialect variants. Some background information is statistical, some is biographical; some is contemporary and some, historical; some summarizes large bodies of research and some relates personal experiences. Background information helps you understand the context for your analysis and explain it to your reader. It is not the subject or focus of your research, but it establishes a factual basis or foundation from which you and your reader can proceed. It is therefore advisable to use background sources that are either familiar to your expected audience and/or will be accepted as valid without an argument..