Here's why:
* "Microphone" describes an object, not a concept. Antonyms typically represent opposite ideas or actions.
* It's not a verb or adjective. Antonyms usually involve words that are opposites in meaning.
However, you could consider these options depending on the context:
* Speaker: This is the opposite of a microphone in the sense that it's the device that *produces* sound, while a microphone *captures* it.
* Silence: This is the opposite of the *sound* that a microphone captures.
* No amplification: A microphone is used to amplify sound, so the opposite would be to have no amplification.
Ultimately, the best "opposite" will depend on what you are trying to convey.