Freesound - The Freesound Project is a collaborative database of Creative Commons licensed sounds.
Thematic - Free Music for YouTube Videos & Creators • No Copyright
Sample Focus - The easiest way to find the perfect audio sample
Music For Makers - Licensing good music has never been this easy.
Moody sampler - A cloud connected audio sampler, including 1000+ instruments
Fugue - Free and premium stock music for your videos, games, and UI