"Most theorists note that music does not consist entirely of sounds. Most obviously, much music includes rests. You might think that silence can function only to organize the sounds of music. One counterargument is that an understanding listener listens to the rests, just as she listens to the sounds (Kania 2010). Another is to provide putative cases of music in which the silences are not structural in the way ordinary rests are. John Cage's 4'33" is frequently discussed, though there is broad agreement that this piece is not silent – its content is rather the ambient sounds that occur during its performance. Anyway, both Stephen Davies (1997a) and Andrew Kania (2010) argue Cage's piece is not music, though on different grounds. Kania considers several other contenders for the label of ‘silent music’, arguing that there are indeed extant examples, most notably Erwin Schulhoff's “In Futurum” from his Fünf Pittoresken, which predates Cage's 4'33" by some 33 years."
From Kania, Andrew, "The Philosophy of Music",
The Stanford Encyclopedia of Philosophy (Fall 2012 Edition),
Edward N. Zalta (ed.)
"Now the Sirens have a still more fatal weapon than their song, namely their silence. And though admittedly such a thing has never happened, still it is conceivable that someone might possibly have escaped from their singing; but from their silence certainly never."