25. A non-transitory computer-readable storage medium having stored thereon program instructions that, upon execution by one or more processors of a system, cause the system to perform operations comprising: implementing an output hidden Markov model (HMM) based speech features generator, wherein the output HMM based speech features generator comprises a first configuration of output HMM state models, each of the output HMM state models having a set of generator-model functions, and wherein the implemented output HMM based speech features generator is trained using speech signals of an output speaker speaking an output language; implementing an auxiliary HMM based speech features generator, wherein the auxiliary HMM based speech features generator comprises a second configuration of auxiliary HMM state models, each of the auxiliary HMM state models having a set of generator-model functions, and wherein the implemented auxiliary HMM based speech features generator is trained using speech signals of an auxiliary speaker speaking an input language; implementing a chimaera HMM based speech features generator that is the same as the output HMM based speech features generator, but wherein (i) the set of generator-model functions of each given output HMM state model of the chimaera HMM based speech features generator is replaced with a particular set of generator-model functions from among the auxiliary HMM state models of the second configuration that most closely matches the set of generator-model functions of the given output HMM, (ii) fundamental frequency (F0) statistics of the chimaera HMM based speech features generator are speech-adapted using an F0 transform that speech-adapts F0 statistics of the output HMM based speech features generator to match F0 statistics of the auxiliary HMM based speech features generator, and (iii) duration statistics of the chimaera HMM based speech features generator are speech-adapted using a duration transform that speech-adapts duration statistics of the output HMM based speech features generator to match duration statistics of the auxiliary HMM based speech features generator.
25. A non-transitory computer-readable storage medium having stored thereon program instructions that, upon execution by one or more processors of a system, cause the system to perform operations comprising: implementing an output hidden Markov model (HMM) based speech features generator, wherein the output HMM based speech features generator comprises a first configuration of output HMM state models, each of the output HMM state models having a set of generator-model functions, and wherein the implemented output HMM based speech features generator is trained using speech signals of an output speaker speaking an output language; implementing an auxiliary HMM based speech features generator, wherein the auxiliary HMM based speech features generator comprises a second configuration of auxiliary HMM state models, each of the auxiliary HMM state models having a set of generator-model functions, and wherein the implemented auxiliary HMM based speech features generator is trained using speech signals of an auxiliary speaker speaking an input language; implementing a chimaera HMM based speech features generator that is the same as the output HMM based speech features generator, but wherein (i) the set of generator-model functions of each given output HMM state model of the chimaera HMM based speech features generator is replaced with a particular set of generator-model functions from among the auxiliary HMM state models of the second configuration that most closely matches the set of generator-model functions of the given output HMM, (ii) fundamental frequency (F0) statistics of the chimaera HMM based speech features generator are speech-adapted using an F0 transform that speech-adapts F0 statistics of the output HMM based speech features generator to match F0 statistics of the auxiliary HMM based speech features generator, and (iii) duration statistics of the chimaera HMM based speech features generator are speech-adapted using a duration transform that speech-adapts duration statistics of the output HMM based speech features generator to match duration statistics of the auxiliary HMM based speech features generator. 30. The non-transitory computer-readable storage medium of claim 25 , wherein the set of generator-model functions for each given output HMM state model comprises a multivariate spectral probability density function (PDF) for jointly modeling spectral envelope parameters of a phonetic unit of the output language modeled by a given output HMM state model, and a multivariate excitation PDF for jointly modeling excitation parameters of the phonetic unit of the output language, wherein the set of generator-model functions for each given auxiliary HMM state model comprises a multivariate spectral PDF for jointly modeling spectral envelope parameters of a phonetic unit of the input language modeled by the given auxiliary HMM state model, and a multivariate excitation PDF for jointly modeling excitation parameters of the phonetic unit of the input language, and wherein the particular set of generator-model functions from among the auxiliary HMM state models of the second configuration that most closely matches the set of generator-model functions of the given output HMM is the particular set for which: (a) the multivariate spectral PDF is computationally nearest to the multivariate spectral PDF of the given output HMM state model in terms of a distance criterion based on one of mean-squared-error (mse) or Kullback-Leibler distance, and (b) the multivariate excitation PDF is computationally nearest to the multivariate excitation PDF of the given output HMM state model in terms of a distance criterion based on one of mse or Kullback-Leibler distance.