Prosody. That's the word I was looking for. Back when I referred to the "unique sound of a language". Back when I said that I prefer my name to be pronounced differently depending upon the language being used at the time.
Mark Liberman attempts to quantify it at the oft-quoted Language Log, which is also the place to go if you want to debunk crazy theories that English needs to replicate the rules of Latin grammar.
According to Not the Nine o Clock News, the hardest words to say in English are Kinda Lingers. I suggest that a cunning linguist could do better.