1. At least one computer storage medium having computer-executable instructions that, when executed by a computer, cause the computer to perform a method comprising: building, based on text, a lattice comprising speech units, wherein each speech unit in the lattice is obtained from a database comprising a plurality of candidate speech units; finding, by the computer in the lattice, a sequence of speech units that conforms to the text; pruning, by the computer from the sequence of speech units, any of the speech units in the sequence that, based on likelihood ratios and a prosody model that was trained using actual speech, are detected to have unnatural prosody, where the prosody model exhibits a bias toward detecting unnatural prosody; iterating, by the computer, the finding and the pruning until completion that is based on a condition selected from a group of conditions comprising: 1) every speech unit in the sequence corresponding to natural prosody, and 2) iterating a maximum number of iterations.