Copyright Release of Rights Obligations for 1970s Interviews

My university has acquired a collection of recordings of interviews conducted with elderly, Black residents of Pontiac, Michigan in the mid-1970s. There are no copyright release of rights forms, which is understandable since that was not yet an expectation or requirement when doing oral history. We're looking for advice about what to do so that scholars, students, and community members can use these interviews. It is extremely doubtful that any of the narrators are still living. When I was faced with a similar situation in the 1990s, the standard was that I had to make a good faith effort to locate either the narrators or their descendants to receive their permission. I'm not sure what the standard is today, or if there is agreement on any standard. If the good faith standard is still operative, how many descendants must one locate to ensure that the family's desires will be respected? We want to do the right thing, but we aren't sure what it would take. Thanks for any help!