Eternity of divine speech means that speech is not a property acquired by God at a moment in time. By contrast, the letters of a human recitation are pronounced successively; an utterance begins and ends, and the reciter’s act takes place in time.
Eternity of the attribute and temporality of its signification are distinct judgments. A teacher’s understanding, for example, is different from the later sound produced in explaining it. This human analogy illustrates a distinction; it does not equate the nature of divine speech with human faculties.
If inner speech were defined as a sequence of generated sounds, the temporal succession would conflict with the claim of eternity. The classical distinction avoids precisely that identification.