I have some experience researching copyrights in multiple countries, and my experience tends to confirm what the experts are saying: for 70-80% of copyrights on material as recent as 1960-1970, the copyright owner is practically invisible. For older material, under he Berne Convention-like rules, it is not in general possible to determine that material is public domain unless the material is at least 150-200 years old (depending on country) OR ... ALL the authors are sufficiently well-known to have death dates in the public record. It's amazing how hard it is to find dates of death even for people listed in "Who's Who" in the relevant period.
Incredibill seems to be assuming the situation that existed in the U.S. before 1963 -- which was truly much simpler for all concerned. But there have been a lot of lawyers undermining the bridge since then.