The definition of sound in itself is troublesome. I understand the human perspective, we are the one's with the self awareness to perceive a sound and categorize it unlike other species (as far as we know). But then you get into the issue of human perception, such as auditory hallucinations. If no one else is able to hear a sound, but you do, was a sound made? No right? but then you bring the topic question in, with the definition of sound, and all of a sudden, auditory hallucinations don't seem that unreal.
What I mean is, if the definition of a sound depends on if some one can perceive it, then an auditory hallucination is a sound even if only one person heard it.
Oh and my definition of sound is in the most general sense, a meaningless noise, vibrations through the air, etc.