Have you ever heard a musician beat out a drum rhythm on a table? Or have you observed that when you walk down the street singing the rhythm of your steps is part of the music?

The natural world is full of sound and rhythm. The noise of interacting things, the rush of waves, heartbeats, breathing, the pounding of branches in thunderstorms, these are all part of a continuum of rhythm, sound, melody which inform, in a very deep way, our own "human music." Everything we understand about what sound is and could be is conditioned by our experience of life.

In the case of your example, I would say it also depends on possibility. One knock -- this might not yet be music. But already three in a rhythmic pattern starts to say something. The person who hears music may be the person who understands how things might continue. Or the person whose hearing and whose senses are so attentive to the world that s/he perceives the syncopation of that knock in the context of the noise of the world, the acorn falling in the moment of silence at the end of the bird's song, the chair falling over at a dramatic pause in a conversation.

But maybe you meant something more concrete than this -- or something less concrete.