In a region known for hundreds of deep narrow canyons and a high magical energy, few have investigated it and come back alive. Some say there are monsters here… and they might be right.
Your players will probably not be trying to make too much noise, as there is a great echo here and they don’t want to alert the monsters.
As you preceed into the canyons, the walls are cooler than you would expect for being exposed to the sun… perhaps the nights here are really cold.
Your players hear an odd “ppppooockt-t-t-t-t-t-t-t-t-t” noise(think: sound of tennis ball hitting wall) that seems to echo around the area. It is like something is bouncing off the rocks. The echo will occur for a while then go away.
If the players are around their campfire….they will hear dozens of pppooockt-t-t-t-t-t-t” noises, like three dozen handball games. It is terribly loud. If they scream at each other (or make a sudden loud noise) the sound will intensify in both volume and tempo.
Someone might feel something bounce off them. It feels like the vibration in your chest when you are standing next to a big drum being banged. Their sword or a metal item vibrates like a tuning fork… or they banged it against metal.
They will eventually go away… and the preternatural silence occurs (actually it is normal silence… but it seems so loud and so still after all that noise and vibration.
If they players travel deeper into the canyons (i.e. don’t get the hint), it will begin again. But this time louder and more dangerous. Now people will begin to pop ear drums. They will take damage from the vibration. Things that are crystal or glass or ceramic will break. If the players are not fairly quiet and beat a hasty retreat, they will soon die from ruptured ears, eyes, and black and blue marks all over their entire body.
The Living Echos of the canyons are living sounds, much the way an elemental is living fire/ earth/ air/ water. They are somewhat intelligent pack animals (akin to wolves). They bounce around the canyons absorbing heat from the rocks (as food). Fires and the warm blooded bodies of animals are also “food”. In fact, fire will create a feeding frenzie.
Certain common sounds create “disharmony” / “discordance” in their bodies causing them a bit of discomfort. They will put up with this as the character bring new heat for their pack to absorb.
Silence spells are the equivalent of walls of flames to paper golems - that is to say, very deadily. Once they encounter this phenomena once, they will avoid it and the casters like the plague… though they may cause rockslides to indirectly attack said caster/ group.
Additional Ideas (1)
Wherever a colony of Unknown Death lives, nearby can soon be found a colony of Hush Moss, a steadily growing carpet of blue moss-like flora. Of special note is the growth of leaf-shaped structures on the surface of the moss itself.
Unlike normal plants, this one doesn't use photosynthesis to generate energy, thus the lack of a green coloration. Much as the Unknown Death consumes heat, this specialized plant consumes sound, making it a natural enemy of the living sounds.
While it can grow anywhere there is a sufficient quantity of sound, generally the only place with enough sound is near a colony of Unknown Death.
Inspired by Manfred's suggestion of a sound-consuming membrane. The natural enemy of these living sounds.