From Wesley Freyer’s Moving at the Speed of Creativity…
The best list I’ve seen lately is Hall Davidson’s, which he has shared as a Word document and linked from his Kit Building page.
Another resource to know about is Acid Planet’s free 8 packs of loops. They share a new 8 pack to download for free every week. I have these and other resources listed on my Free Audio Resources page.
Remind students that audio, music, and even short sound effects are intellectual property just like pictures. More information on copyright and citing sources can be found in the resources and activities in the GenYES and TechYES teacher guides.