12. The voice retrieval method according to claim 8 , further comprising: a feature quantity calculating step of calculating a feature quantity of the sound signal subjected to retrieval in the likelihood obtainment segment specified in the segment specifying step for each frame that is a time window to compare the sound signal with an acoustic model; and an output probability obtaining step of obtaining, for each frame, an output probability that the feature quantity of the sound signal subjected to retrieval is output from each phoneme contained in the phoneme string, wherein in the likelihood obtaining step, respective values that are each a logarithm of the output probability obtained for each frame contained in the likelihood obtainment segment specified in the segment specifying step are added to obtain the likelihood of the likelihood obtainment segment.
12. The voice retrieval method according to claim 8 , further comprising: a feature quantity calculating step of calculating a feature quantity of the sound signal subjected to retrieval in the likelihood obtainment segment specified in the segment specifying step for each frame that is a time window to compare the sound signal with an acoustic model; and an output probability obtaining step of obtaining, for each frame, an output probability that the feature quantity of the sound signal subjected to retrieval is output from each phoneme contained in the phoneme string, wherein in the likelihood obtaining step, respective values that are each a logarithm of the output probability obtained for each frame contained in the likelihood obtainment segment specified in the segment specifying step are added to obtain the likelihood of the likelihood obtainment segment. 13. The voice retrieval method according to claim 12 , wherein: in the converting step, phonemes of an acoustic model that does not depend on adjacent phonemes are arranged to convert the retrieval string into the phoneme string; in the likelihood obtaining step, the likelihood of the likelihood obtainment segment specified in the segment specifying step is obtained based on the phoneme string; the method further comprises: a selecting step of selecting, based on the likelihood obtained in the likelihood obtaining step, candidates of the estimation segment from the plurality of specified likelihood obtainment segments; a second converting step 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 step of obtaining, for each of the plurality of candidates selected in the selecting step, a second likelihood showing a plausibility that the segment selected as the candidate estimation segment in the selecting step is a segment where voices corresponding to the retrieval string are uttered based on the second phoneme string; and in the identifying step, the estimation segment is identified among plurality of candidates selected in the selecting step based on the second likelihood obtained in the second likelihood obtaining step.