The term 'Singing Sentence' is not explicitly defined but the section 'H 3. Clear and Effective Sentences' on sentence structure and effective communication seems the most relevant to understand this concept as it is related to rhythm and song in language.
The section that would tell readers the meaning of a “Singing Sentence” can be deduced from the various given texts. However, the specific term “Singing Sentence” is not directly defined in these excerpts.
The last text snippet referring to diction, utterance, and poetry suggests that it relates to the way language is imbued with rhythm and song, as found in poetry or lyrical prose. Thus, the section labeled 'H 3.
Clear and Effective Sentences' that aims to help one write sentences clearly and succinctly is likely the closest fit, as it discusses sentence structure which is crucial for understanding how to create sentences that “mean language into which rhythm, 'harmony,' and song enter.”