The quantum fields are current in every level in spacetime, even in vacuum areas. After you question "what basically vibrates", I believe the medium you're looking for are these quantum fields. The pitch of a sound is determined by the frequency of vibration (with the sound producing object). Actually, the https://cesarjdxpi.onzeblog.com/28663038/an-unbiased-view-of-mason-industries-vibration-isolators