7. The at least one computer readable storage device of claim 5 , wherein the second cost for determining the prosody modification values includes an absolute frequency likelihood cost, the frequency slope likelihood cost, a frequency linear approximation error cost, and a prosody modification cost.
5. At least one computer-readable storage device encoded with a speech synthesis program which causes a system for synthesizing speech from text to perform: 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; 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 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 program further causing the system to perform the step of increasing the prosody modification cost of continuous speech segments having a slope likelihood greater than a given value in the first speech segment sequence before determining the prosody modification values in response to detection of the continuous speech segments.