In his post Free Loops for Windows Jake Ludington listed some sources of loops and samples that can be used in making your own music. These sources might also offer sounds and music clips you can use in putting together your audio-visual presentations. Here’s what Jake said.
KVR Audio emerged many years ago as an excellent source of drumkits, samples, loops, sound banks and other tools for creating your own music. Many of the downloads are free and free of any complicated rights for commercial use.
Freesound Project is a collaborative collection of sounds and loops released under a variety of Creative Commons licenses. You have to check each track to see whether they are available for commercial use or not, but all tracks are available for non-commercial use.
BeatSuite (obsolete link removed) offers a bunch of free royalty free tracks. The BeatSuite offerings come with a couple of caveats. The tracks are free for non-commercial use. The tracks tend to be lower quality than pay versions of the same loops. Registration is required in order to access free tracks.
Meanrabbit offers a bunch of great sounds you can loop or use to build loops. Many of the sounds included are part of the urban landscape, from trains to telephones, but there are also some fairly unique sounds available too.