1. A voice processing device, comprising: a processor that operates as: a presenting unit that presents text to an operator; a voice acquisition unit that acquires a voice of the operator reading aloud the text; an identifying unit that identifies output intervals of phonemes included in the voice of the operator; a determination unit that determines whether each of time lengths of the output intervals is normal; a frequency acquisition unit that acquires frequency values respectively representing occurrence frequencies of contexts, respectively corresponding to the phonemes, each context including the phoneme and another phoneme adjacent to at least one side of the phoneme; a weight calculator that calculates a weight corresponding to each of the phonemes in accordance with a frequency value of the context; and a score calculator that calculates, as a score representing correctness of the voice of the operator, a value in accordance with a ratio of a first value to a second value, the first value representing a sum of weights corresponding to the phonemes, the second value representing a sum of weights corresponding to the phonemes having the time lengths of the output intervals that are determined as normal.