4'33"

In 1952 John Cage composed a versatile piece for any instrument or ensemble of instruments entitled "Four minutes and 33 seconds." It would be difficult to call it music, strictly speaking, for reasons that are obvious once you read more about it here:
4'33"

Wikipedia describes John Cage as notable for his non-standard use of musical instruments, but it's the disuse of musical instruments that's most notable in 4'33". Played correctly, there should be no sound from the instruments during the piece's duration. The point that Cage was trying to make is that ordinary life is full of a menagerie of sounds and sensations that we rarely pay much attention to, and that by encouraging the audience to sit and contemplate them silently he aimed to make them more mindful of the aural environment around them.

In the event, much of the audience didn't get it and walked out during its first performance.