Reddit reviews Music in the Late Twentieth Century: The Oxford History of Western Music

We censored art too, and for less good reasons. Have your friend go to cia.gov and read about the "Congress for Cultural Freedom." This was a CIA-backed organization that actively promoted avant-garde art as a cold war measure; Jackson Pollock, the famous "abstract expressionist" who literally just threw paint at canvases, was the one of the artists they funded.

Now you might argue that promoting certain kinds of art is different than censoring anything that isn't in the Official Style. But Western artists were under heavy pressure to toe the avant-garde line. McCarthyism, for example, all but forced American composers to write serial music: a heavily mathematized form of composition that was mostly discouraged in the Soviet Union. Music written in more traditional styles became branded as "communist." In Europe, the American-backed Darmstadt Summer School -- which, until very recently, one had to attend in order to be recognized as a composer in the West -- basically condemned anything but the most avant-garde styles as "authoritarian." Aaron Copland, a leftist American composer whose work often drew from folk styles, went from this to this after being called in front of the House Un-American Activities Commission.

Which is to say: art is not apolitical, and it does not take place in a vacuum. From the standpoint of their class interests, the western bourgeoisie were perfectly correct in censoring art that, from their standpoint, was subversive. Similarly, the socialist countries were and are perfectly justified in censoring art that is being used as a wedge to to drive their societies apart. Interestingly, socialist countries' bans on avant-gard artistic styles were never as complete as the Western media likes to claim: in addition to very traditional pieces like Gliere's Heroic March for the Buryat-Mongolian ASSR you had things as modern as Penderecki's Threnody for the Victims of Hiroshima . This actually seems to argue for a greater artistic plurarity than you had in the West; the criteria was not whether art was old or new, but whether it advanced or retarded the socialist cause.

Sources:CIA article on the Congress for Cultural Freedom

Richard Taruskin: Music in the Late 20th Century

If we take the "genre" to be the postwar avant-garde and the postmodern turn, I would point you toward Xenakis and Ligeti as starting examples. Much of their work is "modern" in the basic sense of "make it new" -- but Ligeti's Grand Macabre is theatrical and motley in a way one could link to the postmodernism of Sinfonia.

We could come up with lots more examples, but I would actually recommend books as a more useful source. For example, vol. 5 of Taruskin's History of Western Music, Music in the Late Twentieth Century, will introduce you to many of these composers in a structured way.

(I actually haven't read this, though I mean to. But I've looked at the table of contents, and having read the previous volume and other Taruskin work, I expect it to be useful.)