The Collaborative International Dictionary of English v.0.48:

Polyphone \Pol"y*phone\, n.
   A character or vocal sign representing more than one sound,
   as read, which is pronounced r[=e]d or r[e^]d.
   [1913 Webster]

WordNet (r) 3.0 (2006):

polyphone
    n 1: a letter that has two or more pronunciations; "`c' is a
         polyphone because it is pronounced like `k' in `car' but
         like `s' in `cell'" [syn: polyphone, polyphonic letter]