Similar in meaning:
* Aside: This is a direct address to the audience, often used to reveal a character's thoughts or feelings.
* Soliloquy: A long speech delivered by a character alone on stage, expressing their thoughts and feelings.
* Monologue: A long speech by one character, but not necessarily alone on stage.
* Underbreath: Similar to a whisper, but slightly more forceful.
* Muffled: A softer, more subdued whisper.
* Subvocalized: Spoken quietly, almost inaudibly.
More figurative:
* Murmur: A soft, low, and continuous sound.
* Hiss: A sharp, whispering sound.
* Guttural: A deep, throaty sound.
The best word will depend on the specific context of your writing.