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 converting process of converting a retrieval string into a phoneme string; 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 the phoneme string converted in the converting process; a time length deriving process of deriving, based on the continuous time length obtained in the time length obtaining process, a plurality of time lengths corresponding to a plurality of utterance rates different from one another as candidate utterance time lengths of voices corresponding to the retrieval string; a segment specifying process of specifying, for each of the plurality of time lengths, a plurality of likelihood obtainment segments within the sound signal subjected to retrieval, each of the plurality of likelihood obtainment segments having the time length derived in the time length deriving process; 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 obtaining 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: the processor further executes a selecting process of selecting, based on the likelihood obtained in the likelihood obtaining process, one of the plurality of time lengths; and in the identifying process, the estimation segment is identified among the likelihood obtainment segments with the selected time length based on the likelihood obtained for the likelihood obtainment segment with the selected time length.