well, for cage was into zen-budhism and mushrooms, i don't find it so extremely difficult to understand
plus, he comes to the audiences that widely are city people and throws something they might have already forgotten - an idea of the other world existing around them. there is something else to listen to - not just ''music''. it certainly a question of 'what is music?', but intentionally addressed to modern western man. in other cultures it would not be so much valid and strong. it would not strike people as it strikes modern western cultured man. that is - biting their self-indulged listening practice, which has become not-listening . it's as much philosophical as it is ironic.
and, of course, a step to publically share his views on impossibility of silence.
and, yes, maybe ask, even himself, is sound (and-what sound?) music then?
i certainly don't see it as an answer, but a question, which is the central stone of avant-garde thought in general. like duschamps 'pisuar'.