General synonyms:
* Silent
* Speechless
* Dumb (can be considered offensive)
* Quiet
* Soundless
* Voiceless
Synonyms related to speaking:
* Inarticulate
* Unspoken
* Unvocal
* Tongue-tied
* Close-mouthed
Synonyms related to sound:
* Muffled
* Deadened
* Stifled
* Drowned out
Synonyms related to technology:
* Disabled (for microphones, etc.)
* Turned off
The best synonym for "mute" will depend on the specific context in which you are using it.