Here are a few options:
* Whisper: This is the most direct opposite, meaning to speak very softly.
* Mutter: This suggests speaking quietly and unclearly, perhaps under one's breath.
* Speak softly: This is a general term for speaking quietly and politely.
* Communicate calmly: This emphasizes a lack of emotion or aggression in the way one communicates.
* Silence: This is the complete absence of sound.
The best opposite for you depends on the specific context.