In my eyes (or ears, to be specific), everything is music and can create music. And I really mean just about everything.
I know it wasn't a very popular or well recieved movie, but August Rush has some great examples of this. That kid created a symphony just from hearing a windchime's melody, adding the wind and a whole bunch of other stuff that I can't remember. And when he was in Times Square, everything that made noise was turning into a song, in his ears.
Silence can be considered an instrument, a breath, or yes, even a fart. It goes from there to the physical instruments, like string, brass, and percussion instruments, to the digital ones, like provided in VSTs and soundfonts.
It really depends on how you see and/or hear the world and interpret it that makes you decide what is an instrument and what isn't. That's my personal opinion.