In terms of volume:
* Shout: A loud and forceful utterance.
* Yell: A loud and forceful cry.
* Roar: A deep, loud, and prolonged sound.
* Bellow: To shout loudly and forcefully.
In terms of secrecy:
* Announce: To make known publicly.
* Declare: To state something firmly and publicly.
* Proclaim: To announce officially or publicly.
* Broadcast: To transmit information to a wide audience.
In terms of manner:
* Speak: To communicate verbally, but not necessarily loudly.
* Talk: To engage in conversation.
* Discuss: To talk about something in detail.
The most appropriate opposite will depend on the specific situation in which "whisper" is used.