Focusing on sound amplification:
* Soundproofing: This would be the opposite in terms of its function, as it aims to *reduce* sound rather than amplify it.
* Muffler: This device specifically reduces the noise of engines or other machinery.
Focusing on the direction of sound:
* Earphones/Headphones: These focus sound towards the listener rather than outwards.
* Whisper: A soft, quiet way of speaking, the opposite of shouting through a megaphone.
Focusing on the purpose:
* Communication device that promotes quiet: A device designed for silent communication, like sign language or a communication board.
* A symbol of silence: Something like a "hush" sign or a library.
Ultimately, the best opposite for a megaphone depends on the context and what you're trying to convey.