De Anima

It is not necessary to ask whether soul and body are one, just as it is not necessary to ask whether the wax and its shape are one, nor generally whether the matter of each thing and that of which it is the matter are one. For even if one and being are spoken of in several ways, what is properly so spoken of is the actuality.

But voice is a certain sound of that which is animated; for nothing inanimate emits a voice; but they are said to emit a voice from similitude, as a pipe, and a lyre, and such other inanimate things, have extension, modulation, and dialect; for thus it appears, because voice, also, has these.

Sound is the motion of that which is able to be moved, after the manner in which those things are moved, that rebound from smooth bodies, when any one strikes them. Not every thing... sounds... but it is necessary, that the body which is struck should be equable, that the air may collectively rebound, and be shaken. The differences, however, of bodies which sound, are manifested in the sound, which is in energy; for, as colours are not perceived without light, so neither are the sharp and the flat perceived without sound. But these things are asserted metaphorically, from those which pertain to the touch; for the sharp moves the sense much in a short time, but the flat a little in a long time. The sharp, therefore, is not rapid, and the flat slow; but such a motion is produced of the one, on account of celerity, and of the other on account of slowness, that, also, which is perceived in the touch, appears to be analogous to the acute and obtuse, for the acute, as it were, stings; but the obtuse, as it were, impels: because the one moves in a short, but the other in a long time. Hence it happens that the one is swift but the other slow. Let it therefore be thus determined concerning sound.