1910 can be considered as the time when any and all sound began to be understood as the material out of which music can be made.
It was then that Luigi Russolo began formulating his theory of The Art Of Noises, and Nikolai Kulbin pubished his essay Free Music.

Is is astounding that Varese is given so much credit for introducing sound in to musical composition given Russolo's Art of Noises manifesto, and his diligent
implementation of the ideas in the intonarumori instruments and music for the ensemble.
 
 

"The variety of noises is infinite. If today, when we have perhaps a thousand different machines, we can distinguish a thousand different noises, tomorrow, as new machines multiply, we will be able to distinguish ten, twenty, or thirty thousand different noises, not merely in a simply imitative way, but to combine them according to our imagination."  (Russolo, from Art of Noises", 1914).

Here is a picture of the first page of Kulbin's original 1910 article Free Music:
 
 

(I hope to have a translation of the whole article here soon; the following is from this essay). 'People are organs of a living earth - and the cells of its body. The symphony of the cosmic concert is the music of nature - the natural "free music". If you pay attention to this art and laws of its development, everybody knows that the noises of the sea, wind, thunderstorm, makes a symphony as well as the music of birds - but right now, people exploit the music of nature according to the old laws - if they were paying more attention, they would be enlightened more - It would turn out that water, air and birds, don't sing according to our notes, but using all the notes that they find pleasure in - and with that, the laws of the natural music are observed exactly' (translated from the Russian by Ruslana Zitserman and Lonce Wyse).

Sea View, 1910 Nikolai Kulbin