1. An apparatus for generating at least one phonetic pronunciation for an input sequence of letters selected from a predetermined alphabet, comprising: a memory for storing a plurality of letter-only decision trees corresponding to said alphabet, said letter-only decision trees having internal nodes representing yes-no questions about a given letter and its neighboring letters in a given sequence; said memory further storing a plurality of mixed decision trees corresponding to said alphabet, said mixed decision trees having a first plurality of internal nodes representing yes-no questions about a given letter and its neighboring letters in said given sequence and having a second plurality of internal nodes representing yes-no questions about a phoneme and its neighboring phonemes in said given sequence, said letter-only decision trees and said mixed decision trees further having leaf nodes representing probability data that associates said given letter with a plurality of phoneme pronunciations; a phoneme sequence generator coupled to said letter-only decision tree for processing an input sequence of letters and generating a first set of phonetic pronunciations corresponding to said input sequence of letters; a score estimator coupled to said mixed decision tree for processing said first set to generate a second set of scored phonetic pronunciations, the scored phonetic pronunciations representing at least one phonetic pronunciation of said input sequence.