Sound cannon can largely be defeated with a shield + hearing protection. Place the shield between yourself and the sound cannon and it'll deflect/redirect most of the sound (it is high intensity though, so the added hearing protection is still necessary). It can even return to sender to some degree.

If you want to keep them out of an area, use flanking and skirmishing tactics combined with barricades and area denial. You know the territory, you can be a lot more mobile than they can, and that's how you can force them to retreat (or at the very least divide their attention/break their formation) by making their existing position untenable.

Police still love to use a phalanx style formation on the whole for crowd control, and that leaves them really vulnerable to flanking as the bulk of their force is concentrated at the front, and breaking formation to address skirmishers would leave the front vulnerable as well.

As to barricades and area denial, it depends on what you're trying to stop. If it's advances on foot, anything that can make the terrain difficult to traverse (particularly in formation) will help some amount. If it's vehicles, you don't necessarily have to obstruct the whole roadway, but rather force them to slow down/follow a path you've set for them, and have a plan should they follow it.