You have a large library of sound recordings, including some old speeches in the public domain. What is the best way to treat them with regard to Content ID?

Use a Custom ID to flag these sound recordings as public domain so you can keep an eye on them.
Identify the public domain speeches and don’t enable them for Content ID matching.
Set policies to route all claims for manual review so you can spot claims against public domain content.
Enable your entire catalog for Content ID matching and check your Claims report quarterly.