I'm skeptical that they knew that vibrating air was what our ears perceived as sound. If you know a little about the history of science, there are tons of theories that held up for some time that are no less "ridiculous" to our eyes than some special medium moving through air that our ears process as sound. The fact that the _source_ of sound was known to vibrate doesn't exclude this possibility, and _that's_ what violin/piano players knew about (Sources of visible light often give off heat, but saying that light and heat are the same thing is not implied by that).