The magic-based monster repellent/weapon system that you've described sounds a lot like a modern-day LRAD - a non-lethal weapon that uses highly-focused sound waves to incapacitate or repel a target. Your communication devices generated vibrations in the magical aether at extremely high frequencies, and they could focus these vibrations in a general direction (similar to a directional antenna). The frequency is adjustable and if turned down too low, stops interacting with the aether and interacts with normal matter instead. Interacting with air creates rather intense sound waves. Ages ago, magical practitioners were experimenting with this technology and inadvertently caused several catastrophes. Perhaps they deafened half a village, accidentally found the resonant frequency of the castle's wall and vibrated it to collapse, or perhaps they stumbled across the legendary brown note. After that, this magical technology was removed from use and further study banned.

The monsters that currently plague your kingdom are known to have a highly-developed sense of hearing. They rely on it for hunting, navigation, and communication, like a bat or a dolphin. Some of your soldiers remembered these ancient communication devices and secretly experimented with it. They tuned them down to generate sound at a frequency that monsters are very sensitive to, but is well outside the range of human hearing.

Sound intensity drops with the square of the distance to the target, so getting hit with one of these things at close range is devastating but at a distance is barely noticeable. Focus one in a particular direction and you have your "magic musket", or leave it omnidirectional to create a bubble of sound around you.

Your LRAD devices are powerful enough to induce permanent hearing damage at close range, anything from severe tinnitus to complete deafness. Hearing damage of this magnitude is a death sentence for one of these monsters. They would be partially blind and forced to rely on their under-developed senses of sight and smell, and they would be unable to communicate with each other. Your monsters are intelligent enough that they warn each other about the danger and most will shrink away at the mere sight of one of these devices.

How do your caravans travel safely? The Republic has produced enough of these devices to outfit every caravan with at least one. At least, that's true as far as your monsters know. The Republic has no way to build a functional unit, but duplicating the chassis is a fairly simple process for a blacksmith. Some caravans carry a working unit but most carry a dummy unit. Your troops are trained to never turn one on until a monster is practically within melee range. From a monster's point of view, every attack against a caravan is a game of Russian roulette. There's no way to know if it's real until it's too late to survive the damage.

For those monsters that get a bit too brave, a few chirps from a tuning fork, pitch pipe, or other musical instrument tuned to the same frequency can sound just like the warning shots that your soldiers fire from the real weapons. You can't generate enough sound to cause real damage but again, they can't know that.

The magic-based monster repellent/weapon system that you've described sounds a lot like a modern-day LRAD - a non-lethal weapon that uses highly-focused sound waves to incapacitate or repel a target. Your communication devices generated vibrations in the magical aether at extremely high frequencies, and they could focus these vibrations in a general direction (similar to a directional antenna). The frequency is adjustable and if turned down too low, stops interacting with the aether and interacts with normal matter instead. Interacting with air creates rather intense sound waves. Ages ago, magical practitioners were experimenting with this technology and inadvertently caused several catastrophes. Perhaps they deafened half a village, accidentally found the resonant frequency of the castle's wall and vibrated it to collapse, or perhaps they stumbled across the legendary brown note. After that, this magical technology was removed from use and further study banned.

The monsters that currently plague your kingdom are known to have a highly-developed sense of hearing. They rely on it for hunting, navigation, and communication, like a bat or a dolphin. Some of your soldiers remembered these ancient communication devices and secretly experimented with it. They tuned them down to generate sound at a frequency that monsters are very sensitive to, but is well outside the range of human hearing.

Sound intensity drops with the square of the distance to the target, so getting hit with one of these things at close range is devastating but at a distance is barely noticeable. Focus one in a particular direction and you have your "magic musket", or leave it omnidirectional to create a bubble of sound around you.

Your LRAD devices are powerful enough to induce permanent hearing damage at close range, anything from severe tinnitus to complete deafness. Hearing damage of this magnitude is a death sentence for one of these monsters. They would be partially blind and forced to rely on their under-developed senses of sight and smell, and they would be unable to communicate with each other. Your monsters are intelligent enough that they warn each other about the danger and most will shrink away at the mere sight of one of these devices.

How do your caravans travel safely? The Republic has produced enough of these devices to outfit every caravan with at least one. At least, that's true as far as your monsters know. The Republic has no way to build a functional unit, but duplicating the chassis is a fairly simple process for a blacksmith. Some caravans carry a working unit but most carry a dummy unit. Your troops are trained to never turn one on until a monster is practically within melee range. From a monster's point of view, every attack against a caravan is a game of Russian roulette. There's no way to know if it's real until it's too late to survive the damage. It's much easier to avoid caravans, towns, and other large groups of people and wait for small groups of travelers instead.

The weapons/decoys are quite large, however, and need to be mounted on a decent-sized coach or wagon. Individuals traveling on foot or horseback would have no practical way to carry one with them. Individuals can carry a tuning fork, pitch pipe, or other musical instrument tuned to the same frequency as the weapons. A short chirp can sound just like the warning shots that your soldiers fire from the real weapons, potentially scaring off an attacking monster (potentially a consumable item for escaping fights).