The main reason that I consider vibrations to be sound with or without an observer is that we use the speed of sound in a medium as the fastest rate at which a signal can be transmitted through it mechanically. For instance, we have received several questions about "faster than light" information from people who figured that you can push on the end of a stick here on Earth, who's other end is on Mars, and have that push be instantaneously transmitted. The proper response is that the movement can't propagate faster than the speed of sound in wood. Ignoring the logistical problems regarding orbital mechanics, the lack of a person on Mars doesn't negate the fact that a shift of mass occurs only after a significant delay. That push is "sound" because it involves the transfer of energy along a chain of atoms.
As I was asked in high-school, by someone who I can't remember: If Helen Keller falls over in the forest, does anyone hear her?
edit: Kjamha, I just spotted your last post. Bear in mind that most definitions are not consistent among differing cliques. You will not get the same ones from different bodies of expertise.
