It would seem to me that in this scenario, when it came to the notion of
fair market value, it would be zero.
this is indeed a complicated question. My mentor and friend, sound genius Tony Schwartz, recorded Marshall McLuhan on many occasions. The McLuhan family owned the rights to his "ideas" but Tony Schwartz had the only recording. They had to form a partnership of sorts or either one of them could profit in any way from the comments that McLuhan had made.
My music archive is 40 years old (on 1/4" audiotape) and was recorded for my documentary film. Most experts believe that I have the rights as releases were signed at that time. Without a release, the value of a recording could be nothing unless negotiated with the rights holder