I have to call BS on that one being that in WWI (circa 1914 - 1919) basic audio amplification was all vacuum tube based and loud speaker design was very poor at best and subsonic design was even less understood. That and producing the massive amounts of sound energy needed to stop or at least alter the heart beat in an open air space is still unbelievably difficult and impractical even by todays standards which such equipment would not fit on any truck that was built in the 1914 -1919 era.
Mostly I call BS because LF sound is highly omidirectional and wind change would have little effect on its intensity and direction.