Just because an image or sound is "available" on the Web does not mean that you can freely use that image or sound.
Unless you can identify specific permission for your intended use already posted on the source, you should assume that you'll need to request permission.
Digital Image Rights Computator (Visual Resources Association)
An example of a clear statement about permissions appears on the Social Explorer website: Terms & Conditions of Use for Publicly-Accessible Materials for Non-Subscribers. For example, their content may be made available to "the audience members at a scholarly or educational presentation, lecture, seminar, or similar activity conducted by you (via displays and print hand outs only)." "Content may not be incorporated into an unrestricted database or website."
The Metropolitan Museum of Art's Terms & Conditions page provides answers to questions such as: May I put images or text from www.metmuseum.org on a file server at my school or museum? on my personal website?
The Library of Congress' Prints & Photographs Division provides Copyright and Other Restrictions That Apply to Publication/Distribution of Images: Assessing the Risk of Using a P&P Image.
Wikipedia provides an Image Use Policy.
YouTube provides Copyright information, including:
Google Help provides information about Usage Rights, including information about usage rights filters in their advanced search pages.