General Synonyms:
* Quiet
* Hush
* Mute
* Still
* Noiseless
* Soundless
* Unspoken
* Unvoiced
More Specific Synonyms:
* For people: Taciturn, reticent, uncommunicative, speechless
* For sounds: Muffled, subdued, hushed, dead
* For places: Desolate, secluded, tranquil, serene
The best synonym to use will depend on the specific meaning you want to convey.