12. A system comprising: one or more processors; memory; and machine-readable instructions stored in the memory, that upon execution by the one or more processors cause the system to carry out operations comprising: transforming an audio input signal into a first sequence of feature vectors and a second sequence of feature vectors, wherein both the first and second sequences of feature vectors correspond in common to a sequence of temporal frames of the audio input signal, and wherein each respective feature vector of the first sequence and a corresponding respective feature vector of the second sequence bear quantitative measures of acoustic properties of a corresponding, respective temporal frame of the sequence of temporal frames of the audio input signal, processing the first sequence of feature vectors with a neural network (NN) implemented by the system to generate a NN-based set of emission probabilities for a plurality of hidden Markov models (HMMs) implemented by the system, processing the second sequence of feature vectors with a Gaussian mixture model (GMM) implemented by the system to generate a GMM-based set of emission probabilities for the plurality of HMMs, by computing, for each temporal frame, weighted sums of the NN-based emission probabilities and the GMM-based emission probabilities, merging the NN-based set of emission probabilities with the GMM-based set of emission probabilities to generate a merged set of emission probabilities for the plurality of HMMs, and applying the merged set of emission probabilities to the plurality of HMMs to determine speech content corresponding to the sequence of temporal frames of the audio input signal, wherein the weighted sums are computed according to weights computationally-determined by at least one processor during to a training process that minimizes a computationally-determined difference between computationally-predicted speech in training temporal frames and predetermined speech in the training temporal frames.
12. A system comprising: one or more processors; memory; and machine-readable instructions stored in the memory, that upon execution by the one or more processors cause the system to carry out operations comprising: transforming an audio input signal into a first sequence of feature vectors and a second sequence of feature vectors, wherein both the first and second sequences of feature vectors correspond in common to a sequence of temporal frames of the audio input signal, and wherein each respective feature vector of the first sequence and a corresponding respective feature vector of the second sequence bear quantitative measures of acoustic properties of a corresponding, respective temporal frame of the sequence of temporal frames of the audio input signal, processing the first sequence of feature vectors with a neural network (NN) implemented by the system to generate a NN-based set of emission probabilities for a plurality of hidden Markov models (HMMs) implemented by the system, processing the second sequence of feature vectors with a Gaussian mixture model (GMM) implemented by the system to generate a GMM-based set of emission probabilities for the plurality of HMMs, by computing, for each temporal frame, weighted sums of the NN-based emission probabilities and the GMM-based emission probabilities, merging the NN-based set of emission probabilities with the GMM-based set of emission probabilities to generate a merged set of emission probabilities for the plurality of HMMs, and applying the merged set of emission probabilities to the plurality of HMMs to determine speech content corresponding to the sequence of temporal frames of the audio input signal, wherein the weighted sums are computed according to weights computationally-determined by at least one processor during to a training process that minimizes a computationally-determined difference between computationally-predicted speech in training temporal frames and predetermined speech in the training temporal frames. 14. The system of claim 12 , 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, 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, and wherein computing, for each temporal frame, the weighted sums of the NN-based emission probabilities and the GMM-based emission probabilities comprises: for each pair of a respective feature vector of the first sequence and a corresponding respective feature vector of the second sequence, determining, for each respective state of the multiplicity, a weighted sum of the respective NN-based conditional probability and the respective GMM-based conditional probability.