i certainly don’t know a whole lot about the evolution of psychoacoustics. i would imagine animals developed ears for the smae reason anything evolves anything. there’s useful information to be gleaned from compressions and rarefactions of air molecules, and the ability to glean that information confers an advantage. once you’ve got the organ and the neural pathways to crunch the incoming data, the rest kind of takes care of itself. i think you nailed the “loud=potentially dangerous” connection. that’s a pretty advantageous connection to have programmed in. it would also be advantageous to “know” to be soothed by the sounds of one’s parents take care of him, or to not be able to help adoring the sounds your baby makes when he’s happy. i’d think it pretty unlikely that very much more than that comes preprogrammed, though.

Also, you’re lucky to have friends who ask such thoughtful, probative questions.