1. A voice retrieval apparatus comprising: a processor; a memory that records a sound signal subjected to retrieval; and an output device comprising a screen, wherein the processor executes following processes: a time length obtaining process of obtaining, from a database that stores continuous time length data on a phoneme, a continuous time length for each phoneme contained in a phoneme string of a retrieval string; an utterance rate information obtaining process of obtaining information on an utterance rate; a time length changing process of changing the continuous time length for each of the phonemes obtained in the time length obtaining process in accordance with the information on the utterance rate obtained in the utterance rate information obtaining process; a time length deriving process of deriving, based on the continuous time length for each of the phonemes changed in the time length changing process, an utterance time length of voices corresponding to the retrieval string; a segment specifying process of specifying, as likelihood obtainment segments, a plurality of segments of the utterance time length derived in the time length deriving process in the sound signal subjected to retrieval; a likelihood obtaining process of obtaining a likelihood showing a plausibility that the likelihood obtainment segment specified in the segment specifying process is a segment where voices corresponding to the retrieval string are uttered; an identifying process of identifying, based on the likelihood obtained in the likelihood obtainment process, an estimation segment where, within the sound signal subjected to retrieval, utterance of voices corresponding the retrieval string is estimated, the estimation segment being identified for each of the likelihood obtainment segments specified in the segment specifying process; and a displaying process of displaying on the screen of the output device, the estimation segment identified in the identifying process, wherein in the time length changing process, a continuous time length for each vowel phoneme and a continuous time length for each consonant phoneme in the continuous time length for each of the phonemes obtained in the time length obtaining process are changed at respective change rates.