"Postmodern music" is a maddeningly imprecise idea as a musical concept. So much of what postmodernism has done is react against the project of modernism, and parts of postmodernism are not even doing this: they are not reacting but repudiating the authority of musical history. So, there are really two strains of the "postmodern" music attitude: anti-modernism and post-modernism. Postmodernism embraces, anti-modernism reacts.
Listen to Samuel Barber's second movement in the String Quartet No. 1 in 1936, Adagio for Strings, which is considered one of the greatest masterpieces of the modern classical era. Click on the green link for the full version.
It has been called the "saddest" classical work written in the modern era. With the Nazis coming to power in 1936 and the American Empire on the brink of a new age, the piece was also used in 2001 to commemorate the thousands who were killed in the September 11 attacks.
I heard Adagio for Strings played at a party over the weekend. This was the largest party I've been to, called United States of Consciousness in Seattle. Thousands of diverse creatures in costumes that embraced references all kinds of different sub-cultures. What we all came to be a part of was this party - and beat our heads to Tiësto - because "in trance we trust".
Listen to the Adagio For Strings as 'embraced' by Tiësto...
This nostalgia for the modernist purity and totalizing demonstration of musical mastery is not present in Tiësto's remix of Adagio. It is opposed to strains of modernist drives toward mastery of musical composition. The nostalgia found in Tiësto is distinct from Barber's in that it completely distrusts elitist 'high' and 'low' concepts in art. It doesn't uphold the distinction between varying senses of melodrama and over-sentimentality in art. Tiësto is imitating the superficial appearances of art, through a formula of "remixing" that is supposed to demonstrate the lack of creativity or originality displayed in art.
For example, electronic dance music djs in the "Happy Hardcore" scene and others who remix popular theme songs from Tetris and Wizard of Oz, really do embrace that "superficial" aspect of postmodern music (again, probably not superficial to those who like it). But I don't agree with the claim that postmodernism is superficial so much, because I think that misunderstanding is mostly caused by a generational gap. Generation X thinks Generation Y's music is not creative.
Why does postmodern music remix so much? I don't think it's because of lack of creativity. It's more like a dialogue that cross-generational and cross-cultural. It's what the millenial generation knows how to do so well. YouTube is full of remixes. By remixing something you add another layer of commentary on to it, and someone watching or listening or reading has to figure out what it is that you added to it, and why that's different from what the original was. It's hard to tell what the original of anything is now. But postmodern music embraces the whole of commentary that came before it. Because everything seems so inter-connected, all content seems more like a discussion rather than a private session by yourself.