The Collaborative International Dictionary of English v.0.48:

muted \muted\ adj.
   1. same as quiet; as, the muted atmosphere of a church.
      Opposite of noisy.

   Syn: hushed.
        [WordNet 1.5]

   2. softened; rendered less loud or harsh; -- of sounds and
      instruments which produce sounds; as, muted trumpets.

   Syn: dull, muffled, softened.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

muted
    adj 1: in a softened tone; "hushed voices"; "muted trumpets"; "a
           subdued whisper"; "a quiet reprimand" [syn: hushed,
           muted, subdued, quiet]
    2: being or made softer or less loud or clear; "the dull boom of
       distant breaking waves"; "muffled drums"; "the muffled noises
       of the street"; "muted trumpets" [syn: dull, muffled,
       muted, softened]

Moby Thesaurus II by Grady Ward, 1.0:

105 Moby Thesaurus words for "muted":
   accented, alveolar, apical, apico-alveolar, apico-dental,
   articulated, assimilated, back, barytone, bated, bilabial, blind,
   broad, cacuminal, central, cerebral, checked, close, consonant,
   consonantal, continuant, damped, dampened, dead, deadened, dental,
   dim, dissimilated, dorsal, dull, dulled, flat, front, glide,
   glossal, glottal, guttural, hard, heavy, high, intonated, labial,
   labiodental, labiovelar, lackluster, lateral, lax, light, lingual,
   liquid, low, lusterless, mat, mid, monophthongal, muffled, narrow,
   nasal, nasalized, occlusive, open, oxytone, palatal, palatalized,
   pharyngeal, pharyngealized, phonemic, phonetic, phonic, pitch,
   pitched, posttonic, retroflex, rounded, semivowel, smothered, soft,
   softened, sonant, sordo, stifled, stopped, stressed, strong,
   subdued, surd, syllabic, tense, thick, throaty, tonal, tonic,
   twangy, unaccented, unrounded, unstressed, velar, vocalic, vocoid,
   voiced, voiceless, vowel, vowellike, weak, wide