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

See also: silent

• undefinable
• undefined
• undelete
• undelineated
• undemanding
• undemocratic
• undemocratically
• undemonstrative
• undeniable
• undeniably
• undenominational
• undependability
• undependable
• undependableness
• undependably
• undepicted
• under
• under a cloud
• under arms
• under attack
• under fire
• under it
• under that
• under the circumstances
• under the weather