This is a great gag I came up with this a while ago. I found this megaphone in the dumpster (I spend a lot of time in dumpsters). The built-in microphone was shot, so I wired in a 1/4" jack for an external mic. Then I had another idea, to use a remote microphone. So I had to install a switch so I could turned on for continous duty and walk away.
This halloween I hid it in the bushes some distance from the house but where I could see the trick~or~treaters coming. I starded to just whisper "over here, over here". It was fun to watch them stop and look around, then when they got closer to the bush, I'd then yell and scare the jeebers out them. It's a30 watt megaphone and is loud.
The possibilities are endless; you could use it in a prop and talk to people as they walk by. It's and easy project, build one.