1. A method for enhancement of speech synthesized by a statistical text-to-speech (TTS) system employing a parametric representation of short-time spectral envelope of speech in a space of acoustic feature vectors, comprising: defining a parametric family of corrective transformations operating in the space of the acoustic feature vectors and dependent on a set of enhancing parameters, wherein number of the enhancing parameters in the set of enhancing parameters is less than a dimension of the space of the acoustic feature vectors; defining a distortion indicator of a feature vector or a plurality of feature vectors, wherein the distortion indicator is not modelled directly by the statistical TTS system; receiving a feature vector output by the system; generating an instance of the corrective transformation by: calculating a reference value of the distortion indicator attributed to a statistical model of the phonetic unit emitting the feature vector; calculating an actual value of the distortion indicator attributed to feature vectors emitted by the statistical model of the phonetic unit emitting the feature vector; calculating the enhancing parameter values depending on the reference value of the distortion indicator, the actual value of the distortion indicator and the parametric corrective transformation; deriving an instance of the corrective transformation corresponding to the enhancing parameter values from the parametric family of the corrective transformations; and applying the instance of the corrective transformation to the feature vector to provide an enhanced feature vector.