Audiovisual material that you have found online is likely to be protected by copyright. Using copyrighted material without permission is illegal.
The links that you will find below offer access to websites where internet users share links to audiovisual materials that have been published freely online with the permission of the rights holder. You are allowed to use these materials, provided that you meet the given licence terms.
http://search.creativecommons.org/
Remixable video material
http://www.openbeelden.nl/.en
Open beelden: initiative of the Dutch Institute for Image and Sound (Beeld en Geluid). Includes 1300 older news items.
http://www.opensourcecinema.org
Open Source Cinema: "lets you create your own videos online, remix media that you have on your computer, as well as remix other people’s media"
Remixable photo material
Flickr: Creative Commons – Attribution
Over 27 million photos with a Creative Commons licence. You can do whatever you want with these photos, as long as you mention the source of the publication.
http://www.morguefile.com
Morguefile: Over 200,000 high quality photos that you may reuse, even without acknowledgements, as long as you do not claim to be the creator of the photos.
Remixable sound material
http://ccmixter.org/view/media/home
Dig.ccmixter: "devoted to helping you find that great music, all of which is liberally licensed under a Creative Commons license so you already have permission to use this music in your video, podcast, school project, personal music player, or where ever…"
http://soundbible.com/royalty-free-sounds-1.html
Sound Bible: hundreds of sound effects that you may reuse, providing that you add acknowledgements.