General:
* Quiet
* Hush
* Mute
* Soundless
* Noiseless
* Still
* Unspoken
* Unvoiced
* Speechless
* Wordless
More specific:
* For people: Taciturn, reticent, tight-lipped, mum
* For sounds: Muffled, subdued, hushed, hushed up
* For things: Inactive, dormant, quiescent, motionless
The best synonym depends on the specific situation and the nuance you want to convey.