General:
* Unheard
* Silent
* Mute
* Soundless
* Unsound
* Unintelligible
More specific:
* Faint: When something is too quiet to hear clearly
* Muffled: When sound is obstructed or softened
* Hushed: When something is very quiet, often intentionally
* Imperceptible: When something is so quiet that it's impossible to detect
* Indistinct: When sound is unclear or fuzzy
The best synonym will depend on the specific context in which you're using it.