Impractical was used incorrectly, sorry. I meant that there are so many practical uses as well as tons of uses that serve no purpose other than to troll players (i.e. screaming bloody murder from the forest just outside a players home )
This could potentially be used for an alarm trap as well. Drill a small hole through the trigger of the megaphone and put a pin in it (that keeps the trigger from being pressed). Put rubber bands around the trigger. Partially disassemble the megaphone, pulling the microphone portion out of the housing, extending the wires, and placing it in front of the bell of the megaphone. Tie rope to the pin, place rope wherever so it's tight.
Someone walks through the specific area, pulls the rope, out comes the pin, rubber bands press the trigger which activates it. Because the microphone is now in front of the bell, it creates a feedback loop (very loud).