Kinds of vocal sample
A cappellas are full vocal performances without music, made for remixing. Vocal chops are short slices replayed as rhythms or melodies. Shouts and ad-libs are one-word accents for drops and gaps. Spoken word adds narration or atmosphere, and choirs add size. Each kind is a collection of its own here.
Two rights in every voice
A vocal sample involves the recording and the person whose voice it is, and a sung line also involves the song it comes from. That is why vocals lifted from records, films or interviews are the riskiest samples to release. Vocals recorded by singers for others to use, typically on Creative Commons remix communities, are the safe route.
Credit and licences
Many free vocals are CC BY, which allows a commercial release with a credit to the singer, and a large share are non-commercial. Sound Server shows the verdict and writes the credit line for each, and the bundle download keeps every credit in ATTRIBUTION.md.