A verb includes an event-meaning and a temporal or grammatical relation. If the verb is applied to an event resembling its ordinary action, rhetorical analysis first considers the underlying action or verbal-noun meaning.
“Speaking” ordinarily describes articulated communication by a speaker. When applied to the eloquence of circumstances, one first likens what circumstances reveal to what speech communicates, then expresses the resemblance in the derived verbal form.
The metaphor is called dependent because the derived form follows a more basic semantic transfer instead of constituting an independent transfer at the level of a generic noun.