Because evolution itself is a very slow process. Having said that, the chance of having a continuum of various animals would mean that evolution would have progressed very fast. Because evolution is slow, different species establish themselves for different ecological niches and environments, this enables them to become better through time at a certain environment which then leads to eventual diversification in the number of species.
The two main properties of sound are frequency and amplitude. The frequency of the sound waves determines the pitch of the sound. The amplitude of sound determines the loudness. The quality of sound is affected by the pitch and the loudness.