24. The non-transitory computer-readable medium of claim 23 , wherein the one or more programs further comprises instructions that cause the electronic device to: determine, using a set of acoustic features of each of the additional candidate speech segments for the first target unit and the set of linguistic features of the second target unit, a respective set of predicted acoustic model parameters for each of the additional candidate speech segments for the second target unit; and determine, using the respective set of the predicted acoustic model parameters for each of the additional candidate speech segments for the second target unit and a set of acoustic features of a corresponding additional candidate speech segment for the second target unit, a likelihood score of each of the additional candidate speech segments for the second target unit with respect to each of the candidate speech segments for the first target unit.
24. The non-transitory computer-readable medium of claim 23 , wherein the one or more programs further comprises instructions that cause the electronic device to: determine, using a set of acoustic features of each of the additional candidate speech segments for the first target unit and the set of linguistic features of the second target unit, a respective set of predicted acoustic model parameters for each of the additional candidate speech segments for the second target unit; and determine, using the respective set of the predicted acoustic model parameters for each of the additional candidate speech segments for the second target unit and a set of acoustic features of a corresponding additional candidate speech segment for the second target unit, a likelihood score of each of the additional candidate speech segments for the second target unit with respect to each of the candidate speech segments for the first target unit. 25. The non-transitory computer-readable medium of claim 24 , wherein the one or more programs comprising instructions that cause the electronic device to select the second candidate speech segment to be used in speech synthesis based on the determined likelihood score comprises instructions that cause the electronic device to: determine whether the likelihood score of the second candidate speech segment with respect to the first candidate speech segment maximizes an accumulated likelihood score; and in accordance with a determination that the likelihood score of the second candidate speech segment with respect to the first candidate speech segment maximizes the accumulated likelihood score, select the second candidate speech segment to be used in speech synthesis.