1. Without speaking
   - mutely, wordlessly, taciturnly 
2. Without a sound
   - noiselessly, soundlessly

See also: silent

• low-ceilinged
• low-class
• low-cost
• low-cut
• low-density
• low-density lipoprotein
• low-down
• low-fat
• low-fat diet
• low-fat milk
• low-grade
• low-growing
• low-interest
• low-key
• low-keyed
• low-level
• low-level formatting
• low-level radioactive waste
• low-lying
• low-minded
• low-necked
• low-pass filter
• low-pitched
• low-powered
• low-pressure