When giving a speech, what you look like and how you sound is just as important as what you have to say. Your body language, tone of voice, posture, all contribute to the relative success of your speech. As many powerful speakers know, how forcefully you say something or how charming you are can be just as convincing as the quality of your arguments.
For more information on how to communicate your ideas, see the following links:
"TED is a nonprofit devoted to spreading ideas, usually in the form of short, powerful talks (18 minutes or less)."
TEDtalks cover every area. They are personal and grand, educational and interesting, scholarly and social. They are long form and, for TEDx and other departments of TED, often quite short. Though they mostly follow the same format, they are great examples of speeches for the modern era, show how to incorporate images, emotions, humor, the audience, and how to think about speeches that are both designed to impress a living audience and also package well for classrooms or streaming.
Follow this link to go to the TED homepage, or search for TEDtalks in the Library's Films on Demand Database. There's a good chance you'll find a great speech on your topic.
There are many lists of famous or world changing speeches. Some provide video or audio, though of course many historical speeches could not be recorded in anything other than transcripts. Some examples include: