The thing is that people even Vulcans have different tessituras. Young people have high pitched voices while old people have low pitched voices. If the meaning of a word was based on frequency it would be harder, nay impossible to produce to some people, while easy to others. This would make communication between variegated people a bit of a challenge.In our language. This is an alien language. Why assume it must follow the same rules, when there are already plenty of Earth languages that don't follow the same rules as English?