Have you considered a servo activated air horn? super loud, super light, super annoying. I'm thinking about the cheap air can ones, and you'd have to hack something together to activate it remotely. Or maybe a mechanically driven air raid siren? And, you might research electronic megaphones. Maybe you could use an FM transmitter to send the audio? They make transmitters that are low power for sending signal to your car radio, for instance. You could perhaps hack one to be more high powered? Or, as another commenter suggested, a walkie talkie - but one with an audio output that you could wire into a megaphone.