WordNet (r) 3.0 (2006):

tone down
    v 1: deaden (a sound or noise), especially by wrapping [syn:
         muffle, mute, dull, damp, dampen, tone down]
    2: make less strong or intense; soften; "Tone down that
       aggressive letter"; "The author finally tamed some of his
       potentially offensive statements" [syn: tone down,
       moderate, tame]

Moby Thesaurus II by Grady Ward, 1.0:

102 Moby Thesaurus words for "tone down":
   abate, achromatize, allay, alleviate, assuage, atone, attemper,
   attune, baffle, bank the fire, blanch, bleach, blunt, chasten,
   chord, constrain, control, cushion, damp, dampen, de-emphasize,
   deaden, deafen, decolor, decolorize, dim, diminish, discolor,
   downplay, drain, drain of color, dull, ease, etiolate, extenuate,
   fade, fluff, fume, gentle, keep within bounds, knead, laxate, lay,
   lenify, lessen, lighten, limber, limber up, loosen, mash, massage,
   mat, mellow, milden, mitigate, moderate, modulate, mollify, muffle,
   mute, obtund, pale, palliate, peroxide, play down, plump, pulp,
   put in tune, reduce, reduce the temperature, relax, restrain,
   shake up, slacken, slow down, smash, smother, sober, sober down,
   soft-pedal, soften, soften up, squash, stifle, stop, string,
   subdue, supple, suppress, tame, tarnish, temper, tenderize,
   tone up, tune, tune down, tune up, underplay, voice, wash out,
   weaken, whiten