1. A computer-implemented method comprising: receiving a collection of source texts; identifying a type from a collection of types for each source text, each source text being associated with a particular type; generating, by data processing apparatus, a type-specific language model for each identified type using the source texts associated with the respective type; storing the language models in a computer-readable medium; receiving an audio source file to be processed, the audio source file having a particular type; selecting a particular weighted combination of the language models, wherein weighting the particular weighted combination of the language models depends on the type of the audio source file to be processed; and generating, by data processing apparatus, a text file from the audio source file based on the selected weighted combination of language models.