A whisper or very soft spoken voice, often used to convey something discreetly.

बहुत हल्की बोलचाल या फुसफुसाहट

English Usage: She spoke in a sotto voice so that only he could hear her.

Hindi Usage: उसने बहुत हल्की आवाज में बात की ताकि केवल वह सुन सके।