George Berkeley argues, as he is an idealist, that sound is a sensation and therefore does not exist outside of the mind. For all of you arguing that sound is a vibrative or undulatory motion in the air, is it not absurd that sound in this sense is explained through the idea of motion, an idea that the human faculty of hearing is, naturally, not concerned with? I'll quote some of Berkeley's philosophical dialogue...
Philonous: Tell me Hylas, to which of the senses, think you, the idea of motion belongs: to the hearing?
Hylas: No certainly, but to the sight and touch.
Philonous: It should follow then, that according to you, real sounds may possibly be seen or felt, but never heard...But can you think it no more than a philosophical paradox, to say that real sounds are never heard, and that the idea of them is obtained by some other sense.