The Collaborative International Dictionary of English v.0.48:

Muffle \Muf"fle\, v. t. [imp. & p. p. Muffled; p. pr. & vb. n.
   Muffling.] [Cf. F. moufle a mitten, LL. muffula, OD. moffel
   a muff. See Muff.]
   1. To wrap up in something that conceals or protects; to
      wrap, as the face and neck, in thick and disguising folds;
      hence, to conceal or cover the face of; to envelop; to
      inclose; -- often with up. --South.
      [1913 Webster]

            The face lies muffled up within the garment.
                                                  --Addison.
      [1913 Webster]

            He muffled with a cloud his mournful eyes. --Dryden.
      [1913 Webster]

            Muffled up in darkness and superstition.
                                                  --Arbuthnot.
      [1913 Webster]

   2. To prevent seeing, or hearing, or speaking, by wraps bound
      about the head; to blindfold; to deafen.
      [1913 Webster]

   3. To wrap or fit with something that dulls or deadens the
      sound of; as, to muffle the strings of a drum, or that
      part of an oar which rests in the rowlock; to muffle the
      exhaust of a motor vehicle.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

muffled \muffled\ adj.
   1. same as muted; as, muffled drums; the muffled noises of
      the street.

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

   2. Wrapped up especially for protection or secrecy; as,
      children muffled almost to the eyebrows.
      [WordNet 1.5]

WordNet (r) 3.0 (2006):

muffled
    adj 1: 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]
    2: wrapped up especially for protection or secrecy; "children
       muffled almost to the eyebrows"

Moby Thesaurus II by Grady Ward, 1.0:

77 Moby Thesaurus words for "muffled":
   armored, bated, between the lines, cased, ceiled, cloaked, clouded,
   coated, coped, covered, covert, cowled, cryptic, curtained, damped,
   dampened, dead, deadened, delitescent, dormant, dull, dulled,
   eclipsed, encapsulated, encapsuled, encased, enveloped, enwrapped,
   esoteric, filmed, flat, floored, hibernating, hidden, hooded,
   housed, latent, loricate, loricated, lurking, mantled, masked,
   muted, mystic, obfuscated, obscured, occult, occulted, packaged,
   paved, possible, potential, roofed-in, screened, scummed, sheathed,
   shelled, shielded, shrouded, sleeping, smothered, softened, sordo,
   stifled, subdued, submerged, swathed, tented, under cover,
   under the surface, underlying, unmanifested, veiled, virtual,
   walled, walled-in, wrapped