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.
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. 6. The at least one computer readable storage device of claim 5 , wherein the first cost for determining the first speech segment sequence includes a spectrum continuity cost, a duration error cost, a volume error cost, an absolute frequency likelihood cost, a frequency slope likelihood cost, and a frequency linear approximation error cost.