1. A speech enhancement apparatus that corrects and outputs unclear portions of input speech data, the speech enhancement apparatus comprising: a voiced/unvoiced-boundary-data output unit that determines a separation of voiced/unvoiced of the input speech data and outputs voiced/unvoiced boundary data as phoneme boundary data that splits the input speech data into a plurality of phonemes; a waveform-feature-quantity calculating unit that calculates a waveform feature quantity of the input speech data for each of the plurality of phonemes, the input speech data being input along with the phoneme boundary data, wherein the waveform feature quantity includes at least one of amplitude values, amplitude variation rates, existence or absence of periodic waveforms, of the phonemes, existence or absence of plosive portions of the phonemes, lengths of the plosive portions, existence or absence of aspirated portions that continue after the plosive portions, lengths of the aspirated portions, and phoneme types of the phonemes before and after the phonemes; a correction determining unit that determines a necessity of correction of the input speech data for each of the plurality of phonemes, based on the waveform feature quantity calculated by the waveform-feature-quantity calculating unit; and a waveform correcting unit that corrects a phoneme of the plurality of phonemes which is determined to be corrected by the correction determining unit by using waveform data that is prior stored in a phonemewise-waveform-data storage unit, wherein the waveform-feature-quantity calculating unit includes a speech data splitting unit that splits the input speech data into the phonemes based on the phoneme boundary data, an amplitude variation measuring unit that measures amplitude values, amplitude variation rates, and existence or absence of periodic waveforms of the phonemes, based on the phonemes that are split by the speech data splitting unit, a plosive portion/aspirated portion detecting unit that detects plosive portions and aspirated portions of the phonemes, based on the amplitude values and the amplitude variation rates that are measured by the amplitude variation measuring unit and the input speech data that is split by the speech data splitting unit, a phoneme classifying unit that classifies phoneme types of the phonemes, based on a detection result by the plosive portion/aspirated portion detecting unit, and the amplitude values, the amplitude variation rates, and existence or absence of the periodic waveforms that are measured by the amplitude variation measuring unit, and a phonemewise-feature-quantity calculating unit that calculates a feature quantity for each of the phonemes that are classified by the phoneme classifying unit.