1. A method performed by one or more computers of an automated speech recognition system, the method comprising: obtaining, by the one or more computers of the automated speech recognition system and using a first set of multiple mappings between phoneme sequences and words, a first word-level transcription for a particular utterance spoken by a particular speaker, the first word-level transcription including a first set of one or more words; obtaining, by the one or more computers of the automated speech recognition system and using a second set of multiple mappings between phoneme sequences and words, a second word-level transcription for the particular utterance spoken by the particular speaker, wherein the first word-level transcription and the second word-level transcription are transcriptions of the same particular utterance spoken by the same particular speaker, and wherein the second word-level transcription includes a second set of one or more words that is different from the first set of one or more words, and wherein the second set of multiple mappings is different from the first set of multiple mappings; determining, by the one or more computers of the automated speech recognition system, that the second set of one or more words of the second word-level transcription includes a word that is different from the first set of one or more words of the first word-level transcription and in response to the determination: determining, by the one or more computers of the automated speech recognition system, feature scores based on the first transcription and the second transcription, wherein the feature scores include (i) a first measure of frame alignment between the first word-level transcription and an audio waveform of the particular utterance, (ii) a second measure of frame alignment between the second word-level transcription and the particular utterance, and (iii) data indicating a difference between a length of the first word-level transcription and a length of the second word-level transcription; and inputting, by the one or more computers of the automated speech recognition system, the feature scores to a trained classifier; receiving, by the one or more computers of the automated speech recognition system, an output of the trained classifier that indicates which of the first word-level transcription and the second word-level transcription is more likely to be a correct transcription of the particular utterance; identifying, by the one or more computers of the automated speech recognition system, a mapping in the second set of multiple mappings that causes the second set of one or more words to be different from the first set of one or more words; obtaining, by the one or more computers of the automated speech recognition system, classifier outputs for each of multiple transcription pairs, wherein each of the multiple transcription pairs comprises two different transcriptions, and the identified mapping from the set of mappings causes a difference between the two different transcriptions of each transcription pair; aggregating, by the one or more computers of the automated speech recognition system, the classifier outputs to generate an aggregate score; comparing, by the one or more computers of the automated speech recognition system, the aggregate score to a threshold; based on comparing the aggregate score to the threshold, determining, by the one or more computers of the automated speech recognition system, that the identified mapping likely represents a valid pronunciation; and in response to determining that the identified mapping likely represents a valid pronunciation, providing, by the one or more computers of the automated speech recognition system and as an output of the automated speech recognition system, a transcription determined using the identified mapping.