1. A computer-implemented method comprising: accessing a unit database of acoustic units and, for each acoustic unit, linguistic data corresponding to the acoustic unit; for each acoustic unit: generating an acoustic fingerprint; determining a probability of the linguistic data corresponding to the acoustic unit occurring in a text corpus; and storing data that associates the acoustic unit with (i) the acoustic fingerprint of the acoustic unit and (ii) the probability of the linguistic data corresponding to the acoustic unit occurring in the text corpus; providing at least a portion of the stored data to a finite state transducer training engine that is configured to train one or more finite state transducers that are used in generating speech from text; determining that the unit database of acoustic units has been updated to include one or more new acoustic units that do not have an associated probability of the linguistic data corresponding to the acoustic unit occurring in the text corpus; for each new acoustic unit in the updated unit database: generating an acoustic fingerprint for the new acoustic unit; identifying an acoustic unit that (i) has an acoustic fingerprint that is indicated as similar to the fingerprint of the new acoustic unit, and (ii) has a stored associated probability; and storing data that associates the new acoustic unit with (i) the acoustic fingerprint of the new acoustic unit and (ii) the probability associated with the acoustic unit that has the acoustic fingerprint that is indicated as similar to the fingerprint of the new acoustic unit; and providing at least a portion of the new stored data to the finite state transducer training engine.
6. The method of claim 1 , wherein identifying an acoustic unit that has an acoustic fingerprint that is indicated as similar to the fingerprint of the new acoustic unit comprises: calculating a respective similarity measure between the acoustic fingerprints of the new acoustic units and the acoustic fingerprints of each other acoustic unit in the unit database; determining a nearest acoustic fingerprint to the acoustic fingerprint of the new acoustic units according to the similarity measure; and identifying the acoustic unit associated with the nearest acoustic fingerprint.