The Collaborative International Dictionary of English v.0.48:

Deafen \Deaf"en\, v. t. [imp. & p. p. Deafened; p. pr. & vb.
   n. Deafening.] [From Deaf.]
   1. To make deaf; to deprive of the power of hearing; to
      render incapable of perceiving sounds distinctly.
      [1913 Webster]

            Deafened and stunned with their promiscuous cries.
                                                  --Addison.
      [1913 Webster]

   2. (Arch.) To render impervious to sound, as a partition or
      floor, by filling the space within with mortar, by lining
      with paper, etc.
      [1913 Webster]

WordNet (r) 3.0 (2006):

deafen
    v 1: be unbearably loud; "a deafening noise"
    2: make or render deaf; "a deafening noise" [syn: deafen,
       deaf]
    3: make soundproof; "deafen a room"

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "deafen":
   awake the dead, baffle, blast, blast the ear, boom, crash,
   crescendo, cushion, damp, dampen, deaden, din, dull, fill the air,
   muffle, mute, peal, rattle the windows, rend the air,
   rend the ears, resound, ring, rise, rock the sky, soft-pedal,
   soften, split the eardrums, split the ears, startle the echoes,
   stop, stun, subdue, surge, swell, thunder, tone down