In re-reading Deckman's original post, I don't think there is anything at all wrong with what you are doing. You are not "publishing" these recordings or making them available for all in a library or online site. What you are doing is preserving a personal collection that you made decades ago. Your purpose is the same as it was originally, based on what you posted, and you received permission at the time when the individuals allowed you to record them.   There is no "ethical" question here.

This is very similar to what Virginia noted about her "home" movies and tapes.

Also, I agree with M.Ted concerning the ownership of the tape - but the performer does have a say in how it is used, unless it is expressed before hand. Otherwise, every single bootleg recording would be legal.

Deckman asked about Smithsonian. They do have "rights" issues that they deal with.   Probably the most famous example was the use of an Alan Lomax recording for "Brother Where Art Thou". They tracked down the singer who was recorded on field recording of a chain gang back in the 1950's to present him with a royalty check - a rather substantial one because of how the song was used.   Frankly, many royalites on their recordings are worth pennies and not worth anyones time to collect.   There are tens of dollars to be made in folk music.