From Dark and Darker Wiki
'Stats' page mentions that under Agility, there is "Manual Dexterity" that affects song playing speed. Furthermore, under Resourcefulness, there is "Persuasiveness" that affects song buff time.
Confusingly the info on this page says that Will is used for buff duration instead, and knowledge for song playing speed.
What the hell, what actually affects the bard here? So confusing.