The Collaborative International Dictionary of English v.0.48:

Modulate \Mod"u*late\, v. i. (Mus.)
   To pass from one key into another.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Modulate \Mod"u*late\, v. t. [imp. & p. p. Modulated; p. pr. &
   vb. n. Modulating.] [L. modulatus, p. p. of modulari to
   measure, to modulate, fr. modulus a small measure, meter,
   melody, dim. of modus. See Mode.]
   1. To form, as sound, to a certain key, or to a certain
      portion.
      [1913 Webster]

   2. To vary or inflect in a natural, customary, or musical
      manner; as, the organs of speech modulate the voice in
      reading or speaking.
      [1913 Webster]

            Could any person so modulate her voice as to deceive
            so many?                              --Broome.
      [1913 Webster]

   3. (Electronics) To alter the amplitude, frequency, phase, or
      intensity of (the carrier wave of a radio signal) at
      intervals, so as to represent information to be conveyed
      by the signal; -- a technique used to convey information
      by means of radio waves transmitted by one electronic
      device and received by another.
      [WordNet 1.5]

WordNet (r) 3.0 (2006):

modulate
    v 1: change the key of, in music; "modulate the melody"
    2: vary the pitch of one's speech [syn: tone, inflect,
       modulate]
    3: fix or adjust the time, amount, degree, or rate of; "regulate
       the temperature"; "modulate the pitch" [syn: regulate,
       modulate]
    4: adjust the pitch, tone, or volume of
    5: vary the frequency, amplitude, phase, or other characteristic
       of (electromagnetic waves)

Moby Thesaurus II by Grady Ward, 1.0:

139 Moby Thesaurus words for "modulate":
   abate, accommodate, adapt, adjust, adjust to, allay, alleviate,
   alter, ameliorate, assuage, attemper, balance, bank the fire,
   be changed, be converted into, be renewed, better, blunt,
   bottom out, box in, break, break up, change, chasten, checker,
   chop, chop and change, circumscribe, come about, come around,
   come round, condition, constrain, control, convert, damp, dampen,
   de-emphasize, deaden, deform, degenerate, denature, deteriorate,
   deviate, diminish, diverge, diversify, downplay, dull, extenuate,
   fit, flop, haul around, hedge, hedge about, improve, inflect,
   intonate, jibe, keep within bounds, lay, leaven, lenify, lessen,
   lighten, limit, lower, meliorate, mitigate, moderate, modify,
   mutate, narrow, obtund, overthrow, palliate, play down, qualify,
   re-create, realign, rebuild, reconstruct, redesign, reduce,
   reduce the temperature, refit, reform, regulate, regulate by,
   remake, renew, reshape, restrain, restrict, restructure, revamp,
   revive, ring the changes, season, set, set conditions, set limits,
   shift, shift the scene, shuffle the cards, slacken, slow down,
   smother, sober, sober down, soften, stifle, subdue, subvert,
   suppress, swerve, tack, take a turn, tame, temper, tone down, tune,
   tune down, turn, turn aside, turn into, turn the corner,
   turn the scale, turn the tables, turn the tide, turn upside down,
   undergo a change, underplay, vary, veer, warp, weaken,
   work a change, worsen