The Collaborative International Dictionary of English v.0.48:

Soften \Sof"ten\, v. t. [imp. & p. p. Softened; p. pr. & vb.
   n. Softening.]
   To make soft or more soft. Specifically:
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   (a) To render less hard; -- said of matter.
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             Their arrow's point they soften in the flame.
                                                  --Gay.
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   (b) To mollify; to make less fierce or intractable.
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             Diffidence conciliates the proud, and softens the
             severe.                              --Rambler.
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   (c) To palliate; to represent as less enormous; as, to soften
       a fault.
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   (d) To compose; to mitigate; to assuage.
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             Music can soften pain to ease.       --Pope.
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   (e) To make calm and placid.
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             All that cheers or softens life.     --Pope.
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   (f) To make less harsh, less rude, less offensive, or less
       violent, or to render of an opposite quality.
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             He bore his great commision in his look,
             But tempered awe, and softened all he spoke.
                                                  --Dryden.
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   (g) To make less glaring; to tone down; as, to soften the
       coloring of a picture.
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   (h) To make tender; to make effeminate; to enervate; as,
       troops softened by luxury.
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   (i) To make less harsh or grating, or of a quality the
       opposite; as, to soften the voice.
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WordNet (r) 3.0 (2006):

softened
    adj 1: toned down
    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:

84 Moby Thesaurus words for "softened":
   abject, apologetic, bated, chastened, complaisant, conditioned,
   constrained, contrite, controlled, creamy, damped, dampened, dead,
   deadened, delicate, dull, dulled, easy, eggshell, flat, gentle,
   gloss, hedged, hedged about, humble, humbled, hushed, in control,
   in hand, iridescent, leavened, light, limited, low-key, mellow,
   mellowy, melted, mild, mitigated, modified, modulated, mollified,
   mother-of-pearl, muffled, muted, nacreous, nonresistive, nonrigid,
   opalescent, pale, pastel, patinaed, pearly, penitent, penitential,
   penitentiary, qualified, quelled, quiet, repentant, restrained,
   restricted, sad, seasoned, semigloss, sheepish, simple, smothered,
   sober, soft, soft as putty, soft-colored, soft-hued, somber, sordo,
   stable, stifled, subdued, subtle, sweet, tempered, tender, touched,
   whisper-soft