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"