1. A method comprising: receiving an audio input; processing the audio input using an acoustic model to generate a respective phoneme score for each of a plurality of phoneme label sequences; processing one or more of the phoneme scores using an inverse pronunciation model to generate a respective grapheme score for each of a plurality of grapheme label sequences; and processing one or more of the grapheme scores using a language model to generate a respective text label score for each of a plurality of text label sequences, wherein each text label score reflects a likelihood that the audio input is represented by the corresponding text label sequence.