Humans have an audible hearing range between 20Hz and 20000Hz, but sound waves do exist below and above this range. Sound waves below 20Hz we cannot hear, but we can feel. The pressure of the sound wave is still there but our ears are simply not equipped to hear it. This pressure can still affect our body though. The low frequency waves cannot just vibrate our ear drums but our entire bodies. This could potentially cause devastating effects when they are at very high levels.
There was a Russian born French researcher named Vladimir Gavreau who supposedly did research into the effects of infrasound (below our audible range) and has results which ranged from people being sick to having their internal organs liquefied when exposed to an infrasonic whistle for prolonged periods of time. However this is widely considered to be a conspiracy theory.
Theoretically, however, it is certainly possible to create acoustic weapons. The low frequencies of infrasound mean it has a very long wavelength which can either diffract around objects or move through a body, creating an oscillating pressure system. This can cause different parts of the body resonate which can have extremely unusual effects. For example there have been cases where people have supposedly seen ghosts, but in reality it was just a fan resonating the room at exactly 18.98Hz, the exact resonant frequency of the eyeball, causing them so see grey shapes.
Every different material in the body will have a different resonant frequency. At these frequencies the material will stretch and contract with the low frequency wave. But if the level of that wave was at huge levels, it can cause the material to move too much and potentials move, break, or even explode. As the frequency of the sound is too low for us to hear, the only experience there will be is the pressure wave on the body. However, because most of the important parts of the body, such as the brain or organs, are surrounded by fluid, the levels of the infrasound would have to be insanely high. The liquid acts as a dampener and so can reduce the level of the sound affecting the targeted body part by 70dB or more. At which point it would just be slightly annoying the targeted person. Therefore the level of the sound to cause destructive resonance would have to be at least 240dB which is highly impractical and not worth the effort to create.
Therefore I’m afraid to say that a handheld sonic weapon that can make people heads blow up is highly unlikely to ever exist. Acoustic weapons can be good deterrents, such as the LRAD, but not so good as potential death rays.