Orphan works
Copyrighted works whose rights holders cannot be identified or located, making it impossible to obtain a license. Orphan works create a legal barrier for those who wish to use them legitimately. Various jurisdictions have introduced or proposed frameworks to allow limited use of orphan works under specific conditions.
Articles about Orphan works

Music Rights Clearance: The Complete Process for Licensing Permissions
Music rights clearance is the operational work that turns a creative cue into lawful use—and it routinely breaks projects without a repeatable process. This guide gives a step-by-step, reference-style workflow to identify required rights, map ownership, execute licenses, and document compliance for common scenarios including sync, samples, covers, reissues and digital uses.

Sync Licensing Royalties: A Complete Guide to Synchronization Rights and Payments
Sync placements generate multiple, overlapping revenue streams and frequent confusion about who gets paid, when, and through which society or channel. Treat this as sync licensing royalties explained for implementers: it breaks down one-off sync fees versus master licenses, public performance and mechanical flows, cue-sheet requirements, and the metadata and identifiers your systems must track.