From Unofficial Handbook of the Virtue Universe
The loudest possible sound that can be created at one standard atmospheric pressure (equal to about 101.325kPa at sea level) is somewhere in the neighborhood of 194dB. This occurs when the air pressures in the "peaks" and "valleys" of the sound wave measure twice normal atmospheric pressure and zero pressure respectively. Sounds louder than 194dB are only possible through mediums other than air or at air pressures higher than normal, since the "valleys" of the sound wave cannot have negative pressure. (For reference: 40dB is the level of a normal conversation, 70dB will cause hearing loss, and 165dB is the sound that a space shuttle makes when taking off; if one sound is 3dB more powerful than another, it is twice as loud.)
A consequence of atmospheric pressure is that approximately fourteen pounds of pressure is exerted on every square inch of one's body at any time from the air, but as these many pounds of pressure push in from all angles one ordinarily can't detect it. If these fourteen pounds per square inch were to vanish or quickly diminish on one side of one's body, however, a substantial force would be felt as coming from the opposite side. If said force would stem from, say, a sound wave oscillating at A4 (440 Hz) then this force would be felt as switching between both sides of one's body four hundred times a second. Needless to say, this would cause extreme trauma to an unprotected human body.
Most rigid materials (including, in many cases, human bone) can be demolished with a loud-enough sustained tone at their precise resonant frequency, and most buildings contain at least one "weak spot" frequency: a frequency that, when played in proximity to the structure loudly or for prolonged periods of time can cause extreme vibrations in supports and severe structural damage. Nearly anything can be at least affected by a powerful enough blast of a low-frequency sound wave, and the human body is no exception. Beyond causing intense headaches, sound at high enough levels can cause shifting of internal organs against each other which in turn can cause internal bleeding. Less powerful sounds cause feelings of unease and tension, nausea, and stress, and if a wave is emitted at the precise resonant frequency of one's eye (usually a hair above 19 Hz) it can create visual distortions and minor hallucinations.
Very few phenomena, natural or unnatural, are capable of producing sounds at precise enough frequencies or loud enough volumes to have effects such as these, however.