Based on function:
* Whisper: This emphasizes the opposite of amplification.
* Microphone: This is the opposite in terms of direction of sound: a microphone gathers sound, while a megaphone amplifies and directs it.
* Silence: This emphasizes the absence of sound.
Based on size/volume:
* Small speaker: This indicates a device that produces sound, but on a smaller scale.
* Earphone: This is a device that focuses sound on a single listener, the opposite of broadcasting to a crowd.
Based on purpose:
* Private communication: This emphasizes the opposite of public address.
* Quiet conversation: This focuses on the opposite of shouting.
The best antonym will depend on the specific context and the nuance you want to convey.