FIrst, let's define "modern". Better still, let's accept that few members would end up agreeing on any such definition, so let's make one up, just for the sake of convenience. For the purpose of this specific exercise, let's assume "modern music" to mean "music by living composers" (and even then I am immediately aware that this would identify Carter non Pann's Pocahontas from the later 1930s as "modern", which already seems to undermine even that arbitrary definition). The sheer diversity of styles, media, techniques etc. that have been developed and explored during those last 70 or so years are what most potently reveal that there's no such thing as "modern music" about which one can speak or write in any useful way. Four important piano sonatas were composed in the immediate aftermath of WWII, Carter's, Dutilleux's and the first two by Boulez; just look how different they are from one another - even the two Boulez works might be thought to be separate by far more years than is the case. Musique concrète and the earliest vestiges of electronic music were being hatched at the same time, while Shostakovich was having one of his longest pauses between symphonies and Britten had not long completed Peter Grimes. And that was pre-1950; look at what has happened since. We have Stevenson and Xenakis, Sorabji and Stockhausen, Rubbra and Ferneyhough, Carter and Glass, Finnissy and (George) Lloyd, James Dillon and James MacMillan, all working contemporaneously - and I have omitted literally thousands of others working in many different ways. This is why it is impossible to construct any meaningful definition of "modern music" that will be acceptable to and accepted by all. I think what may be alluded to here is "unfamiliar music", of which, however, there is plenty from pretty well every years of the last eight or so centuries for most people to get to know. Case closed, peut-être?...
Best,
Alistair