These helms were originally forged for the royal guards of the western kingdoms. They are full helms made out of iron. They are very ornate and are identified with the code or arms for the king that commissioned thier building. The effective range for these helms is about one mile; obstacles like walls and mountains do not seem to interfere with the reception.
The Royal guards would don the helms and coordinate their attack on the roving bands of thieves or whomever they were out to get. The helms proved useful when they wished to remain unseen and still be able to flank a target at the most opportune time for them.
The helms only transmit and receive when worn. The down side to these helms is that they transmit all sounds at the same volume as the original. If Dirth the assassin is wearing on of the helms and his partner Silious is wearing a matching helm and within a mile of Dirth then Dirth will broadcast the sounds from Silious’s location and vise versa. You can see how having one person wearing a helm and sitting in a noisy area would make another person wearing a helm who is trying to sneak up on someone have a hard time trying to be quite.
The exact magic’s used to allow the helms to transmit to each other have been forgotten for centuries.
These helms have the ability to transmit anything at the same volume. They only work when worn, and do not have a volume control. Only Helms with the same code of arms will transmit and receive to each other.