For present purposes we will stick to sources that are public domain, Creative Commons, or otherwise licensed for reuse.
Free Music Archive: “It’s not just free music; it’s good music”. Mostly Creative Commons licensed materials.
Soundly – Soundly lets you drag sound effects from the cloud and drop them into your project.
freesound.org – massive and diverse, if unwieldy, collection of samples.
archive.org – audio – This library contains over two hundred thousand free digital recordings ranging from alternative news programming, to Grateful Dead concerts, to Old Time Radio shows, to book and poetry readings, to original music uploaded by our users. Many of these audios and MP3s are available for free download.
Wikimedia Commons (Sounds) – A massive, if at times unwieldy collection of open source sounds. (Usually in .ogg or .wav format).
soundcloud.com – (legal note on cc) – “To search for Creative Commons tracks, perform a basic search on SoundCloud, and then filter the results by Sounds only. Then, filter the results again so that you can find sounds to use or modify commercially. Here’s a screenshot to make sure you’re on the right track: http://bit.ly/14tM4j3”
Macaulay Library of bird sounds at Cornell University. Since 1915!