There are legitimate (IMHO) uses for these. For example, a crowd panics and people start getting trampled. An LRAD can be used to communicate directions quickly at long range to help stop the panic. The report sounds like the police were doing what they should be doing. Have the organizers of that March complained about police actions or disputed the report? If not, then I would tend to believe the report. If correct…
You're making the same mistake that everyone in this thread is afraid the cops are also making. A sound cannon is not a loudspeaker, it's a weapon. If you want a loudspeaker, get a loudspeaker. This thing is designed to inflict actual, serious, permanent damage to people mechanically, from a large standoff distance. It's used to paralize combatants approaching a convoy. It shouldn't be used against crowds at home.
[1] https://www.lradx.com/wp-content/uploads/2016/06/LRAD-Produc...