6. The method of claim 1 , wherein the plurality of HMMs collectively comprise a multiplicity of states, wherein generating the NN-based set of emission probabilities for the plurality of the HMMs comprises: for each respective feature vector of the first sequence, determining, for each respective state of the multiplicity of states, a respective NN-based conditional probability of emitting the respective feature vector of the first sequence given the respective state, and wherein generating the GMM-based set of emission probabilities for the plurality of the GMMs comprises: for each respective feature vector of the second sequence, determining, for each respective state of the multiplicity of states, a respective GMM-based conditional probability of emitting the respective feature vector of the second sequence given the respective state.
8. The method of claim 6 , wherein each of the HMMs in the plurality is associated with a respective elemental speech unit, and has one or more states corresponding to one or more temporal phases of the associated, respective elemental speech unit, wherein the multiplicity of states comprises a collection of the one or more states of each of the HMMs in the plurality, and wherein determining the speech content corresponding to the sequence of temporal frames of the audio input signal comprises determining a probable sequence of elemental speech units based on a most likely sequence of states of the multiplicity.