THE FOURTH DEVELOPMENT [of new music], which is in many ways the most interesting, [is] namely the replacement of tones by sounds, and musical by acoustical hearing. Varèse, Pierre Schaeffer and their immediate successors awoke composers and audiences to the many new sounds, some of them produced electronically, that could enter the space of music without destroying its intrinsic order. These experiments are not what I have in mind when referring to the replacement of tones by sounds and musical by acoustical hearing. I am thinking of a more general transition, from Tonkunst to Klangkunst, to use the German expressions – a transition of deep philosophical significance, between two ways of hearing, and two responses to what is heard.
The author is arguing that we've moved from lyrical experimentation with tones, to serialization (complete lyrical structure of tones, probably commonly known as musical theory), and now to the avant garde of replacing tones (chords, progressions, structure) with recorded/processed sounds. Sounds (presumably) lack the melodic/harmonic qualities such as notes played together in a classical scale or progression.
This leads to: The experience of ‘next,’ and the inevitability of the next, has been chased away. In a concert devoted to music of this kind the audience can know that the piece is ended only because the performers are putting down their instruments.
Naturally we can fill in the blanks if we hear 75% of a chord progression, but with sounds, we have no idea what is coming next. They're disjointed.
I'm not sure what the author's conclusion is, but this seems to be as close to a summary as he gives: It seems to me that, if there is, now, to be a music of the future it will, in that way, belong with the music of the past.
Personally, if I hear a bunch of sounds that my brain can't connect together, and that don't please me or move me, I would have trouble calling that acoustical experience "music".