From the FAQ: "scores published in the last 85 years are mostly copyrighted and therefore unavailable in our corpus" [ http://www.peachnote.com/info.html ]. The corpus comes from http://imslp.org/ , a site mostly devoted to out of copyright scores (e.g. "classical" music).

Mostly unrelated, but if you're interested in hip hop sample sources, check out http://www.the-breaks.com/ . For example, here are uses of the Amen break: http://www.the-breaks.com/search.php?term=winstons&type=...