where to get found sounds
The Freesound Project provides a variety of sounds all for free, and these are all in .wav and .aiff format (so the file quality is good)
Sounddogs has a massive selection of good sounds too
Freesfx is ok and quick to use - scroll down past the blue sounds to the free orange ones.
where to get found music
Jamendo has an excellent archive of music available under creative commons, you may have to pay a small amount if you intend to distribute your film, but the quality here is pretty high.
The Audionetwork offer good rates for what they call student productions.
And for more mashed up mixes try funkyremixes and ccmixter
mac users can turn .wavs into .aifs in iTunes, and PC users can turn aiffs into .wavs with audacity
