1. An acoustic modeling system, comprising: under control of one or more computing devices configured with specific computer-executable instructions, receiving a plurality of characteristics of utterances to be used to create an acoustic model; for each characteristic in the plurality of characteristics: identifying an utterance within a corpus of utterances having the characteristic; and associating at least a portion of the utterance with a tag indicative of the characteristic; receiving an identification of a desired training utterance, wherein the desired training utterance comprises a first portion associated with a first desired characteristic and a second portion associated with a second desired characteristic, and wherein the desired training utterance is not included in the corpus; selecting, from the corpus, a first utterance, wherein a portion of the first utterance comprises at least the first portion of the desired training utterance, and wherein the portion of the first utterance is associated with a tag corresponding to the first desired characteristic; extracting the portion of the first utterance from the first utterance; selecting, from the corpus, a second utterance, wherein a portion of the second utterance comprises at least the second portion of the desired training utterance, and wherein the portion of the second utterance is associated with a tag corresponding to the second desired characteristic; extracting the portion of the second utterance from the second utterance; concatenating the portion of the first utterance with the portion of the second utterance to generate the desired training utterance; and training an acoustic model, wherein: the acoustic model comprises statistical representations of possible sounds of subword units; and the statistical representations are generated based on a comparison between audio data associated with the desired training utterance that is generated and a textual transcription of the desired training utterance that is generated.