5. A computer-readable storage medium having computer-executable instructions stored thereon that when executed by a computer cause the computer to perform steps comprising: receiving text of a word for which a phonetic pronunciation is to be added to a speech recognition lexicon; receiving a representation of a speech signal produced by a person pronouncing the word; converting the text of the word into at least one text-based phonetic sequence of phonetic units; generating a speech-based phonetic sequence of phonetic units from the representation of the speech signal; placing the phonetic units of the at least one text-based phonetic sequence and the speech-based phonetic sequence in a search structure that allows for transitions between phonetic units in the text-based phonetic sequence and phonetic units in the speech-based phonetic description; and selecting a phonetic pronunciation from the search structure, wherein the selected phonetic pronunciation comprises phonetic units of the speech-based phonetic sequence that differ from phonetic units of the at least one text-based phonetic sequence and phonetic units other than phonetic units of the speech-based phonetic sequence.
5. A computer-readable storage medium having computer-executable instructions stored thereon that when executed by a computer cause the computer to perform steps comprising: receiving text of a word for which a phonetic pronunciation is to be added to a speech recognition lexicon; receiving a representation of a speech signal produced by a person pronouncing the word; converting the text of the word into at least one text-based phonetic sequence of phonetic units; generating a speech-based phonetic sequence of phonetic units from the representation of the speech signal; placing the phonetic units of the at least one text-based phonetic sequence and the speech-based phonetic sequence in a search structure that allows for transitions between phonetic units in the text-based phonetic sequence and phonetic units in the speech-based phonetic description; and selecting a phonetic pronunciation from the search structure, wherein the selected phonetic pronunciation comprises phonetic units of the speech-based phonetic sequence that differ from phonetic units of the at least one text-based phonetic sequence and phonetic units other than phonetic units of the speech-based phonetic sequence. 10. The computer-readable storage medium of claim 5 , wherein generating a speech-based phonetic sequence of phonetic units comprises: generating a plurality of possible phonetic sequences of phonetic units; using at least one model to generate a probability score for each possible phonetic sequence; and selecting the possible phonetic sequence with the highest score as the speech-based phonetic sequence of phonetic units.