15. The system of claim 12 , wherein the set of generator-model functions for each given source HMM state model comprises a multivariate spectral probability density function (PDF) for jointly modeling spectral envelope parameters of a phonetic unit modeled by a given source HMM state model, and a multivariate excitation PDF for jointly modeling excitation parameters of the phonetic unit, and wherein determining for each given source HMM state model the particular target-speaker vector from among the target set that most closely matches parameters of the set of generator-model functions of the given source HMM comprises: determining a target-speaker vector from among the target set that is computationally nearest to parameters of the multivariate spectral PDF of the given source HMM state model in terms of a distance criterion based on one of mean-squared-error (mse) or Kullback-Leibler distance; and determining a target-speaker vector from among the target set that is computationally nearest to the multivariate excitation PDF of the given source HMM state model in terms of a distance criterion based on one of mse or Kullback-Leibler distance.
16. The system of claim 15 , wherein the multivariate spectral PDF of at least one source HMM state model has the mathematical form of a multivariate Gaussian function.