Stock.XCHNG is a great site for stock photos. The site has numerous images that are available for free as well as many images that are available for purchase at a low price. The rights for each image are shown, and the descriptions make it very easy for you to determine if the particular image you want to use is available and is free. I found the image below on the site here, and I think this might be a possibility for something to include in the banner for the title of the page.
Since my site will emphasize Social Studies, I did a Google search for "royalty free images + american history". The first hit I got was for U.S. Government Photos and Images | USA.gov which is a link to a collection of numerous government sites. Many of the images on U.S. government sites are in the public domain and are available to be used. While I have used U.S. government sites as sources of information and images in the past, I had never seen this particular list. This is a very helpful link to have so many resources in one place. I found the video below on the history of the U.S. Capitol from the U.S. Capitol web site in their video gallery. I think this would be an excellent video that a teacher could incorporate into a history or government lesson plan.
Finding audio was a bit more of a challenge for me because it was harder for me to find resources that I wanted to use and then figuring out how to get the file into my page. I did get some good ideas for things that would be great assets to include for a Social Studies web site. Speechs, oral histories, musical recordings from different time periods are just a few examples. I found this recording of John F. Kennedy's famous speech that he gave in Berlin in 1963 on Wikipedia's Featured Sounds page and decided to use that.