But let's take another look at music. There are certainly aspects of music that show a definite progression from less complex to more complex or from simple and limited to more ample and extensive. For example, a great deal of what we know of the music of the early Middle Ages consists of simple, one voice melodies that we call "Gregorian Chant" used by the Catholic Church:
Scholars, when discussing this or any other musical form or genre, cannot avoid using the word "development" which implies progress over time. For example, the Wikipedia article on Gregorian Chant linked to above describes it like this:
Gregorian chant developed mainly in western and central Europe during the 9th and 10th centuries, with later additions and redactions. Although popular legend credits Pope St. Gregory the Great with inventing Gregorian chant, scholars believe that it arose from a later Carolingian synthesis of Roman chant and Gallican chant.The origins are obscure, as they are with many musical genres, but it most certainly developed over time. You could argue that the changes were not a progression in an aesthetic sense, but you could also argue that they were. Simply melodies were refined and varied and honed over time. Most of all, added to the single voice of the earlier repertoire, were additional voices to create a polyphonic texture and ways were devised to write this down. We also see a similar progression over a larger field if we look at the technical resources of music. Without confusing notation with practice (it is certain that polyphonic, chordal choral music was sung long before it was written down), we can, by examining the history of what was written down, see a definite accretion of more and more potential resources for composers. As long as we avoid making the crude claims of a linear progression over time, I think that we can certainly see development in the history of music.
What I mean by "crude claims of a linear progression" is the obviously erroneous idea that music written in 1800 is always better than music written in 1700. Absurdities like this are why it is often said that there is no progress in music. But if we fine-tune it a bit, we might say something like in 1800 composers commonly used a wider harmonic palette than composers did in 1700. This is obviously true. While there are some interesting eccentricities like the chromatic harmony of Gesualdo, in general, while there are waves back and forth, there does seem to be a progression from a more limited, consonant harmonic space to one that accepts a higher level of dissonance and more remote modulation. Again, with the recognition that the development is not linear. The late Baroque was far more likely than the Classical to use minor keys, for example, and the wild harmonic ventures of C. P. E. Bach are more extreme than those of the later Joseph Haydn.
But while recognizing these truths, we must also see what are the unmistakable development of resources. Take meter, for example. There were certain metric devices that were common currency in the 17th and 18th centuries. An example would be hemiola that I wrote about here. This is the relatively simple, though very effective device of changing the subdivision within a measure so as to change the feeling of the beat. An example would be to write three half notes in a measure of 6/4 to change the feel to 3/2. Or you could turn two measures of 3/4 into one measure of 3/2. This was often used to set up a cadence. It is still used a lot in Spanish music. But a composer working now has an incredible range of possibilities: hemiola is still available, of course, but irregular meters like 5/4 are also in common use and then there are the complexities of meter that we find in Stravinsky, the "phasing" and other devices in the music of Steve Reich, developments of hemiola with subdivisions of 3+3+2 in Philip Glass and the much more complex metric modulation as is found in the music of Elliot Carter.
Now, of course, there is no guarantee that access to a wider variety of technical possibilities will result in a better piece of music--often the opposite is true! But we can certainly speak of a general tendency of development. As long as we are careful to compare apples to apples and not to oranges, we can look at, for example, the harmony of Haydn, Beethoven, Schubert and Brahms and see that there is a development of wider possibility.
But considering the history of music, or any art, purely from an aesthetic standpoint, we should be very hesitant to claim anything more than merely local progress. Over the greater span, any attempt to claim that Beethoven is on a higher level than Bach aesthetically, or Bach than Josquin, or Stravinsky than Mahler, is simply going to dissolve into fractious debate. What each of these composers set out to do was so different that straightforward comparisons cannot be made. Some things really are incommensurable.
Here is a chromatic madrigal by Carlo Gesualdo:
And here is a consonant work by Arvo Pärt: