CLASSICISM:
Music is a noble craft concerned solely with the expression of beauty and humanity.

ROMANTICISM:
Music is a great art concerned primarily with the passions, tribulations and aspirations of people and nations.

MODERNISM:
Music is potentially any collection of sounds exhibiting structure and expressing ideas.

POSTMODERNISM:
Music is a cultural practice of organized sound serving as an agent of expression and control.

Postmodernism is doomed, for no more significant reason than that it followed Modernism, to be defined by it
(i.e. in its negative space), compared to it (see below), and even examined within its characteristic structures (such as in the whimsical and simplistic historical comparison survey above).  But it in my view it is less interesting to argue that Postmodernism’s rejection of structuralism and grand narrative is a reaction against Modernism than to view it as a necessary and logical step in just such an anathemic theory of historical progression.