1. A method comprising: processing a set of source sounds to generate a source feature vector and processing a set of target sounds to generate a target feature vector; aligning the source feature vector with the target feature vector to generate a joint variable; estimating a probability density function for the joint variable, the probability density function including a mean vector; and training, by one or more processors, a mixture model based on the joint variable by a process that includes: selecting a mixture mean pair from the mean vector, deriving a source spectral envelope and a target spectral envelope for the selected mixture mean pair, and generating a mixture specific warping function for the selected mixture mean pair based on the target and source spectral envelopes.