I'm not a fruit scientist. Fruit does not ripen after being picked.
OK, why is nearly all fruit COMMERCIALLY harvested, always picked "green" ?? I'm surrounded by fruit, nearly year round. It always has that "green" tang, just before it gets ripe. Pick fruit early to ship it.
All I can say, is, I can pick the best melons, regardless of species, right at the vine. In the store, you have to settle for the best of the lot.
Last time I was in Florida, I got a couple mush melons, and, my Daughter could not tell the difference. I purposely picked a "Green" one, smell means nothing, and we compared, after getting home. I had gotten the best from the stand, and she saw it.
A person needs to have several to work with, to learn the art.
Ykick said it well, a DULL sound is dead ripe, getting hollow inside. A SOLID sound is green. It's the same as putting different levels of water in identical glasses. Tap each one and you get a different sound-tone- plunk, whatever you want to call it.
All the non-believers can come down and get free lessons.