Sound is a concept created by brain activity (thoughts). Without a thought to create the concept of sound, sound will not exist to the observer. Sound or sound-waves is nothing more than compressions of air molecules. Some compressions are inaudible to the human ear, and even by instrumentation, yet the compressions must exist as part of the continuum spectrum. Just because it cannot be measured (evidence) does not mean that the compressions do not exist.
The truths we have direct evidence about largely pertain to the so-called visible world. However, there is more truths in the so-called invisible world, which we cannot gather evidence about unless we, and measuring instruments, also become invisible. The reason being is that the invisible (the finer realms) contain all the visible (denser realms). Where the visible cannot contain all that is invisible. In other words, the visible universe exists within a much larger invisible universe (so-to speak).
As I said, far more truths exist in the invisible than anything we can have evidence about in the visible.