1. A method for synthesizing a target voice, the method comprising: receiving symbolic input descriptive of an utterance to be synthesized; selecting one or more portions of the utterance to be constructed from prototype speech units of a target voice corpus, the target voice corpus including speech units recorded from a human speaker, the target voice corpus configured to provide characteristics of the target voice; applying adaptations to selected ones of the prototype speech units of the target voice corpus, to produce adapted units that are contextually appropriate for the utterance; obtaining at least some speech units from a source other than the target voice corpus; and concatenating at least the adapted speech units from the target voice corpus and the speech units from the source other than the target voice corpus to produce a speech waveform for the utterance.