Birds singing near a road

Catalog candidates come from named CC0, public-domain, and CC-BY sources. Open uploads and other archive material are fingerprint-screened against more than 150 million commercial recordings; a match means the file is withheld. Qualifying public-domain recordings instead use documented source authority and publication-date eligibility, or source-curator attestation. The specific recording matters, not just the age of its song.

Each eligible download includes a signed certificate recording the named source, rights basis, clearance-path evidence, required attribution, and a tamper-evident audit chain. That gives creators and collaborators evidence they can inspect independently instead of a label that merely says “royalty-free.” It reduces uncertainty; it is not a legal guarantee.