The session singer who did "The Great Gig in the Sky" vocals on The Dark Side of the Moon famously got £30. She later sued and got an undisclosed settlement.
Also on that album of course "There is no dark side of the moon, really. As a matter of fact, it's all dark". Wasn't that just some random studio guy, on the spur of the moment, too? Simpler times, more creativity.
Pink Floyd wrote some questions down on note cards and asked random people in the studio, recording their answer. The Beatles were part of this, but their answers were too guarded and weren't used.
https://en.m.wikipedia.org/wiki/Clare_Torry