Have you ever thought of impossible sounds or identified such?
An impossible sound is a sound that's (or at least is perceived as being) impossible to record or create and it may also be a sound that we're not sure whether it even exists (i.e. an impossible sound may be "physically impossible", so we'd have to think of it as a some kind of "metaphysical sound" or e.g. a sound that can be only created by complex artificial e.g. computational means).