Update: we have the detector, we want to make it louder.
The detector is assumed to work like "Sting":
Sting has the magical ability to detect orcs or goblins nearby, glowing blue in their presence,
So, we now need something to react to the blue light.
We get our trusted glass vial, but now we tinge it brown and fill it with a silver halide solution, in which the Sting is placed.
As soon as the Sting glows blue, the silver halide will decompose and the pH of the solution will decrease. This can be used to trigger several possible reactions, from the classic litmus test (which has no advantages - you still need to look at it) to non-reversible electrochemical reactions (unfortunately, too weak to be useable - you couldn't drive an electromagnet and build a ringer) or even explosive ones. Hydrogen gas development is easy, and its pressure could be used to audibly "pop" a fragile enough diaphragm.
The amplification is one-shot: after it has been triggered, the solution and any irreversible component has to be replaced.
Pocket watch size and shape, YALDA can alert a user in the presence of certain dangers, like Orcs and Goblins.
Ok, so we need:
- for Orcs and Goblins to emit something - vibrations, chemicals, electromagnetic radiation (including heat),
- for that something to be Orc- and/or Goblin-specific,
- for that something to influence something else, our "detector", under not too awkward conditions,
- for the detector to be of size, shape, resistance etc. suitable for pocketizing.
I think that the least difficult possibility is going with chemistry. Orcs and goblins give off a particular chemical that is otherwise rare. It is well known that both Orcs and Goblins give off a characteristic milk-curdling pong, but a less well known fact is that they also give off a odour that humans cannot smell. This, too, is otherwise strong enough to precipitate a chain reaction whose results can be observed.
The "Orc detector" is then a stoppered glass vial three quarters filled with a clear, bluish liquid. To use, unstopper the vial and wave the stopper in the air three or four times, then plug it back and shake the resealed vial for some seconds. If nothing happens, the odour intensity is less than 0.05 orcs/fathom (one orc at about 37 m is enough to trigger the device; two orcs can be detected at double the distance; orcs' hygienic habits and diet as well as recent rains unto unprotected orcs might impair detection).
If small white specks of snow appear in the liquid, an orc is nearby. The intensity of the snowfall gives a rough estimate of orc number and distance.
When the liquid turns completely clear, the chemicals are exhausted and the detector must be recharged/regenerated.
Another type of detector relies on the orcs being first-generation orcs from the Forbidden Plains, where the horizon is said to glow blue at night and the air itself is poisonous, bringing wasting diseases, blindness and child and cattle malformations.
It is again a stoppered glass vial in which two thin gold foils are hung from a copper hinge, which is driven through the stopper. You rub a glass and copper wand against a wool strip, then touch it to the outer copper knob until the two gold foils suddenly jump away from each other. Take notice of their position and keep the vial still. If the foils stay rigid, you're safe. If they slowly go limp and hung together, there's Orcish stuff nearby - it could be just the buried remains of a campsite, or a tomb, or a corpse. The detector is non-directional.
Orcs and goblins born and raised outside their ancestral homeland do not trigger the detector.
This detector is actually an expedient Lauritsen electroscope. It works as long as the Orcs give off enough radiations. It might be that Orcs are actually genetically mutated humans.