General:
* Quietly
* Softly
* Whispering
* Muttering
* In a low voice
More specific:
* Subdued: Implies a controlled and restrained tone.
* Hushed: Suggesting a sense of secrecy or reverence.
* Muffled: Indicating difficulty hearing due to distance or obstruction.
* Sotto voce: An Italian phrase meaning "under the voice," used to describe speaking very quietly, often in a secretive or conspiratorial way.
The best word to use will depend on the specific context.