Can the facts literally speak for themselves, or is that phrase figurative? I'm unsure, because I'm not sure whether 'speak' or 'speak for' always involves speech.
In the OED entry for 'speak' (subscription required) (there is no entry for the idiom):
- a. fig. Of things: To be expressive or significant; to make some revelation or disclosure.
This is figurative. However, 15a seems to cover it
- a. To indicate, denote, or betoken; to reveal, make known. 1856 R. W. Emerson Eng. Traits iii. 40
The solidity of the structures..speaks the industry of ages
But that is transitive, and I think that there's no direct object in that phrase.
However, there is also an entry to "speak for"
- To indicate; to betoken.
1832 Philol. Museum I. 335 The great mass of evidence that speaks for an intimate affinity between the Pelasgians and the Hellenes.
So it could be literal, but says something slightly weird, that the facts indicate or are in some sense a sign of themselves, perhaps their being facts.