themilum, if you are talking about the old "riddle" that begins "If a tree falls in the forest..." then I agree it is semantic, and hangs on the words "hear" and "sound".

If no one is there to "hear" it, then no one's auditory apparratus transforms the sound energy into nerve impulses to be perceived by the brain as "sound". Do the vibrations still travel through the air if there is no device there to measure them? Rephrased, the question becomes quite banal: "Can you know something has happened without having any evidence whatsoever?" No.