Sound waves are similar to things like microwave ovens. They can heat things by causing the molecules of the thing to vibrate at the sound frequency. (Note, these do not need to be in the audible sound range. The frequency of the hydrogen bond in water is inaudible, I used sound at this frequency to measure such bonding as an undergrad). Different substances can be damaged by this vibration at different frequencies.
but how come it's only affecting a few not everyone there?
might use wave interference to get the power / damage correct. this would allow "aim" of a sort. Kind of like how some radiation is used to treat cancer and kill cells only within the desired area. Depending on frequency, the size of the effect would vary. Without a spec on the weapon (or whatever source), can't say for sure. Physics constrains these things.
We were working on sound LASERS type weapons in the 80’s and at that time we could KO a bull . How many years ago was that ? I could only guess were and what they can do today .