1. A speech synthesis system for synthesizing speech from text, the system comprising: a speech segment database configured to store a plurality of speech segments; means for determining a first speech segment sequence corresponding to an input text, by selecting speech segments from the speech segment database-according to a first cost calculated based at least in part on a statistical model of prosody variations; means for determining prosody modification values for the first speech segment sequence, after the first speech segment sequence is selected, by using a second cost calculated based at least in part on the statistical model of prosody variations, wherein the first cost is different from the second cost; and means for applying the determined prosody modification values to the first speech segment sequence to produce a second speech segment sequence whose prosodic characteristics are different from prosodic characteristics of the first speech segment sequence, wherein the second cost includes at least a prosody modification cost, the system further comprising means for increasing the prosody modification cost of continuous speech segments having a slope likelihood greater than a given value before determining the prosody modification values in response to detection of the continuous speech segments in the first speech segment sequence.
4. The speech synthesis system according to claim 1 , wherein the statistical model uses a decision tree and Gaussian mixture models.