As for protection from weaponized infrasound, it could require a full-body metamaterial shield to deflect all incoming infrasound. And microwaves as we know require a faraday cage to dissipate them, but even then there can be some penetration near the boundary of the cage.
But I guess that could even be the point. Make it dangerous to protest, but avoid images of firing into the crowd, can blame the protestors for "killing each other" or so.
Serbia has been Russia's closest ally for decades, with Russia having its side in the wars of the 90s. Perhaps such a weapon would have been provided to support the current pro-Moscow government.
Alternatively (and more likely) the Serbian government could've simply bought an LRAD device from the United States: https://en.wikipedia.org/wiki/Long-range_acoustic_device
Which
https://www.reddit.com/r/serbia/comments/1jchks6/novi_snimci...