Although it is unwise to take a developmental view of an art like music -- as if Beethoven were an advance over Josquin, or Miles Davis an advance over Louis Armstrong -- there are ways in which it makes sense to talk about musical knowledge growing over time. We certainly know more about certain sounds than was known five centuries ago; that is, we understand how sounds that earlier composers avoided can be used effectively in musical compositions. For example, we now know how the interval of the third, which is considered dissonant, can be used in compositions to create consonant musical phrases.
The main conclusion of the argument is that our understanding of music can improve over the course of time.