1. A digital watermark embedding device comprising: one or more processors; and a memory storing instructions that, when executed by the one or more processors, performs operations, comprising: outputting a synthesized voice according to an input text and phoneme-based alignment regarding phonemes included in the synthesized voice; estimating whether or not a potentially risky expression is included in the input text, and outputting a potentially risky segment in which the potentially risky expression is estimated to be included; associating the potentially risky segment with the phoneme-based alignment, and deciding and outputting an embedding time for embedding a watermark in the synthesized voice; and embedding a digital watermark in the synthesized voice at a time specified as the embedding time for the synthesized voice, wherein the estimating includes outputting a degree of risk of the potentially risky expression that is included in the potentially risky segment, the associating includes setting an embedding strength of the digital watermark based on the degree of risk and outputting the embedding strength, the embedding includes embedding the digital watermark in a sub-band of the synthesized voice based on the embedding strength, the sub-band including at least two neighboring frequency bins, and the embedding further includes embedding a digital watermark bit based on a difference in summed amplitude spectrum intensity between different sub-bands satisfying a threshold.