1. A method performed by one or more computers, the method comprising: obtaining a first transcription for an utterance; obtaining a second transcription for the utterance, wherein the second transcription is different from the first transcription; determining one or more feature scores based on the first transcription and the second transcription; inputting the one or more feature scores to a trained classifier; receiving an output of the classifier that indicates which of the first transcription and the second transcription is more likely to be a correct transcription of the utterance identifying a mapping in the second set of mappings that causes the second transcription to be different from the first transcription; obtaining 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 second set of mappings causes a difference between the two different transcriptions of each transcription pair; aggregating the classifier outputs to generate an aggregate score; comparing the aggregate score to a threshold; and based on comparing the aggregate score to the threshold, determining whether the identified mapping likely represents a valid pronunciation.