The sites listed below are a great place to start searching for music and sound clips. The subjects available will vary from site to site so be sure to check more than one if you can't find what you are looking for.
IMPORTANT: Some of these sites allow the people who created these sound files to place extra restrictions on how their music and sounds are used. Be sure to check the usage rights for each individual file. This is espcially important if you only want to use a short segment of the sound file.
NOTE: Remember, copyright applies to both the piece of music/sound and the performance of that music/sound. Just because one element, such as the performance, is free to use may not mean that there are no copyright restrictions on the other element.