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)