The Collaborative International Dictionary of English v.0.48:

Voiceless \Voice"less\, a.
   1. Having no voice, utterance, or vote; silent; mute; dumb.
      [1913 Webster]

            I live and die unheard,
            With a most voiceless thought, sheathing it as a
            sword.                                --Byron.
      [1913 Webster]

   2. (Phon.) Not sounded with voice; as, a voiceless consonant;
      surd.
      [1913 Webster]

   Voiceless stop (Phon.), a consonant made with no audible
      sound except in the transition to or from another sound; a
      surd mute, as p, t, k.
      [1913 Webster] -- Voice"less*ly, adv. --
      Voice"less*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

voiceless
    adj 1: produced without vibration of the vocal cords; "unvoiced
           consonants such as `p' and `k' and `s'" [syn: unvoiced,
           voiceless, surd, hard] [ant: soft, sonant,
           voiced]
    2: deprived of the rights of citizenship especially the right to
       vote; "labor was voiceless"; "disenfrenchised masses took to
       the streets" [syn: disenfranchised, disfranchised,
       voiceless, voteless] [ant: enfranchised]
    3: uttered without voice; "could hardly hear her breathed plea,
       `Help me'"; "voiceless whispers" [syn: breathed,
       voiceless]
    4: being without sound through injury or illness and thus
       incapable of all but whispered speech [syn: aphonic,
       voiceless]

Moby Thesaurus II by Grady Ward, 1.0:

116 Moby Thesaurus words for "voiceless":
   accented, alveolar, anaudic, aphasic, aphonic, apical,
   apico-alveolar, apico-dental, articulated, assimilated, back, bad,
   bare, barytone, bereft, bilabial, breathless, broad, cacuminal,
   central, cerebral, checked, close, consonant, consonantal,
   continuant, dental, denuded, deprived, destitute, dissimilated,
   dorsal, dumb, dumbfounded, dumbstricken, dumbstruck, empty, flat,
   front, glide, glossal, glottal, guttural, hard, heavy, high,
   inarticulate, innocent, intonated, invalid, labial, labiodental,
   labiovelar, lateral, lax, light, lingual, liquid, low, mid,
   monophthongal, mum, mute, muted, narrow, nasal, nasalized,
   null and void, occlusive, open, oxytone, palatal, palatalized,
   pharyngeal, pharyngealized, phonemic, phonetic, phonic, pitch,
   pitched, posttonic, retroflex, rounded, scant, semivowel, short,
   shy, soft, sonant, speechless, stopped, stressed, stricken dumb,
   strong, surd, syllabic, tense, thick, throaty, tonal, tongue-tied,
   tongueless, tonic, twangy, unaccented, unrounded, unstressed,
   velar, vocalic, vocoid, voiced, vowel, vowellike, weak, wide,
   wordless