The smart approach in 1975 was to use Parsons code, which was also turning songs into hashes, computable in your head. You could then find your song back as simply as looking a word in a dictionary. Hopefully this idea won't die any time soon.
That requires identifying the melody, which is certainly not something all humans can do, and was probably not generally doable by a machine in 1975. It also throws away a huge amount of information, and requires starting from the beginning of the melody.
[1]: https://en.wikipedia.org/wiki/Parsons_code