What ccMixter is
ccMixter is a long-running community remix site, started by Creative Commons, where singers and musicians publish a cappellas, stems and remixes under Creative Commons so others can build on them.
Best for
A cappellas and vocal stems for remixes, plus instrument stems and loops.
Licence and what it means for a release
Licence per upload: CC BY, CC BY-NC, CC0 and the retired Sampling+ family are common. Many a cappellas are non-commercial, so check the verdict before a release; CC BY vocals need a credit to the singer.
How Sound Server searches it
Through ccMixter's public query API, with each upload's licence, tempo where given and download links.