What is an antonym for the word megaphone?

There isn't a perfect antonym for "megaphone" in the sense of a direct opposite. However, depending on the context, you could consider these words:

Based on function:

* Whisper: This implies a quiet and private way of speaking, the opposite of a megaphone's loud and public amplification.

* Microphone: This focuses on capturing sound rather than amplifying it.

* Muffler: This reduces sound, the opposite of a megaphone's amplification.

Based on size:

* Earphone: This is a small device used for listening, the opposite of a megaphone's size and purpose.

Based on purpose:

* Silencer: This is used to reduce noise, the opposite of a megaphone's purpose of amplifying sound.

Ultimately, the best antonym depends on the specific situation you're describing.