I thought we'd end up agreeing except for maybe the "speaking" thing.
Mmm... I still say that if a regenerate infant can -- literally, not "metaphorically" -- leap for joy in the presence of his Savior, then the Bible does teach that infants can "communicate" in some manner on a rudimentary moral level. And so, I believe that Psalm 58:3 teaches that the natural communications of (fallen, unregenerate) Man are sinful from Infancy.
But, nevermind -- that's not really the main issue. It is enough to say that Psalm 58:3 does teach that Infants are Sinners.