Here are some sound resources. If you find other great sites, please add a comment and enter their address:
An interesting collection of sounds that can be used to learn audio techniques and some ambient noise samples http://www.ee.columbia.edu/~dpwe/sounds/
Dig.CCmixter has free to use music samples from projects http://dig.ccmixter.org
Freesound has a large collection of sound effects https://freesound.org
Wikipedia has a collection of links to resources https://en.wikipedia.org/wiki/Wikipedia:Free_sound_resources