12. The computer-implemented method of claim 9 , wherein the first transcription was generated using a limited speech recognizer, the limited speech recognizer comprising a speech recognizer that includes a language model that is trained over a limited speech recognition vocabulary that includes one or more terms from a voice command grammar, but that includes fewer than all terms of an expanded grammar; and wherein the second transcription was generated using an expanded speech recognizer, the expanded speech recognizer comprising a speech recognizer that includes a language model that is trained over an expanded speech recognition vocabulary that includes all of the terms of the expanded grammar.
12. The computer-implemented method of claim 9 , wherein the first transcription was generated using a limited speech recognizer, the limited speech recognizer comprising a speech recognizer that includes a language model that is trained over a limited speech recognition vocabulary that includes one or more terms from a voice command grammar, but that includes fewer than all terms of an expanded grammar; and wherein the second transcription was generated using an expanded speech recognizer, the expanded speech recognizer comprising a speech recognizer that includes a language model that is trained over an expanded speech recognition vocabulary that includes all of the terms of the expanded grammar. 13. The computer-implemented method of claim 12 , further comprising: determining that a particular word or phrase was generated by the limited speech recognizer at a grammatical position within the particular utterance that indicates the particular utterance comprises a voice action command.