If the owner of a copyright doesn’t care to commercially exploit their work, they won’t cover the ongoing expenses to police infringements by submitting takedown requests. There is basically a blacklist of works whose copyrights are valuable enough to cover the bill for submitting takedown requests. This blacklist is self maintaining – there doesn’t need to be a central registry.
The gray list is works which are not policed. Some are in the public domain or under a permissive license like one from Creative Commons. Some are in copyright but not being actively exploited, including orphan works whose owners can’t be reached, as well as barely-exploitable works whose owners can’t be bothered.
Anybody can find out which is which: post a given recording in a visible location and see whether you get a takedown request.