General:
* Whispers
* Murmurs
* Mumbles
* Hushed
* Softly spoken
* Low-voiced
* Under their breath
* In a low tone
More specific:
* Coos (especially for babies)
* Sighs (suggests sadness or resignation)
* Sighs (can also suggest longing or contentment)
* Purrs (implies a soothing or comforting tone)
* Croons (suggests a low, melodic voice)
* Sings (implies a melodious, perhaps even musical, voice)
Figurative:
* The sound of a gentle breeze
* A whisper on the wind
The best word to use will depend on the specific context and the desired effect.