Language is always about the situation and about people involved. Regarding pragmatics, "...the utterance does not suffice to itself." All language is dialogue; all language has a "pragmatic" purpose. (7-8). In language, there are "four cooperative principles: quantity, quality, relation, and manner." In action, all statements are performatives.
Strictly speaking, the book doesn't seem about voice at all, but about poems as speech acts. But sentences seem to have more voice to the degree that we think of them as speech acts, as sentences said by one person to another in a context to make something happen.
By violating the rules of speech acts (e.g., exhorting readers to do something they acknowledge as distasteful), "Poetry... often seems to represent a sort of conspicuous futility in language." 22. Analysis in terms of pragmatics is one of the strongest ways of showing that literary language is often crucially different from "normal language" (which obeys these laws). (R. A. York, The Poem as Utterance, London and NY, Methuen, 1986)