Socrates at least endorses the claim that knowledge necessary for virtue, or (perhaps equivalent) can't be reliably virtuous without knowledge (e.g. Euthypro: can't reliably tell what is pious and what isn't without knowing what piety is).
Maybe also endorses the stronger claim that knowledge sufficient for virtue.
Disputing the necessity claim
The objection: courage is a virtue, but courage seems to be about doing. So courage seems to represent a challenge to Socrates' "intellectualized" view of virtue. "Knowing" about courage seems to consist just in acting courageously. So knowledge not necessary for virtue.
Possible Socratic response: Someone who often acts courageously does not (necessarily) display courage; if he's acting by, e.g., instinct, then really, he's getting lucky.
Disputing the sufficiency claim
The objection: if I have a test, and it's reliable, so I know what the courageous thing to do is, isn't there still a gap between my knowing it and doing it?
Response: First, Socrates must endorse the sufficiency claim, otherwise his emphasis on knowledge looks mysterious. He can't just abandon it. So akrasia seems to be impossible--nothing [internal?] can come between knowledge of right thing to do and the doing of it. A sort of argument for this can be extracted from the Protagoras: seems to be based on an identity of pleasure and the good; the good is pleasure; so pleasure, e.g., can't interfere with the good; being overcome by pleasure just is being overcome with the good! So, again, it's lack of information about pleasure that gets in the way of good action. [Presumably pleasure is the obvious candidate for something that can come between knowledge of the good and good action.]
Complication: the argument structure is weird, though--e.g. in the Protagoras, the argument conclusion is supposed to be negative: it shows that, e.g., Protagoras' view that courage is not reducible to knowledge is false. So it's not a positive argument for the sufficiency claim.