Similar in meaning:
* Murmured
* Muttered
* Hushed
* Softly spoken
* Sotto voce (Italian, meaning "under the voice")
* In a low voice
* In a breathy voice
* In a confidential tone
* In a secretive way
More figurative:
* Breath
* Sigh
* Rustle (for a very quiet sound)
* Sussurrated (formal, literary)
The best synonym will depend on the specific situation and the desired effect.