6. The voice retrieval apparatus according to claim 5 , wherein: the processor further executes: a converting process of arranging phonemes of an acoustic model that does not depend on adjacent phonemes to convert the retrieval string into the phoneme string; a selecting process of selecting, based on the likelihood obtained in the likelihood obtaining process, candidates of the estimation segment from the plurality of specified likelihood obtainment segments; a second converting process of arranging phonemes of a second acoustic model that depends on adjacent phonemes to convert the retrieval string into a second phoneme string; and a second likelihood obtaining process of obtaining, for each of the plurality of candidates selected in the selecting process, a second likelihood showing a plausibility that the segment selected as the candidate estimation segment in the selecting process is a segment where voice corresponding to the retrieval string is uttered based on the second phoneme string; and in the likelihood obtaining process, the likelihood of the likelihood obtainment segment specified in the segment specifying process is obtained based on the phoneme string; and in the identifying process, the estimation segment is identified among plurality of candidates selected in the selecting process based on the second likelihood obtained in the second likelihood obtaining process.
7. The voice retrieval apparatus according to claim 6 , wherein in the selecting process, the plurality of candidates of the estimation segment is selected by selecting, from the likelihood obtainment segments beginning in a predetermined selection time length, the likelihood obtainment segment one by one with a maximum likelihood for each of the predetermined selection time lengths in the likelihood obtainment segments specified in the segment specifying process.