to speak in a low or barely audible voice

फुसफुसाकर बोलना

English Usage: She mutters under her breath when she is frustrated.

Hindi Usage: जब वह निराश होती है, तो वह फुसफुसाकर बोलती है।

a low or indistinct sound made while speaking

खुसर-पुसर

English Usage: There was a soft mutter from the crowd as they awaited the announcement.

Hindi Usage: घोषणा का इंतजार करते समय Crowd में खुसर-पुसर हो रही थी।