1. A computer-readable memory storing computer-executable instructions that, when executed, cause one or more processors to perform acts comprising: performing formant-based frequency warping on fundamental frequencies and linear predictive coding (LPC) spectrums of source speech waveforms in a first language to produce transformed fundamental frequencies and transformed LPC spectrums; generating warped parameter trajectories based at least on the transformed fundamental frequencies and the transformed LPC spectrums; and producing transformed target speech waveforms with voice characteristics of the first language that retain at least some voice characteristics of a target speaker using the warped parameter trajectories and features from target speech waveforms of the target speaker in a second language.