Silence. Hacking music (which as an adult is really the same as studying)?
Metal–preferably melodic, at 130bpm. Which usually means it is Swedish and it is Death Metal.
Don’t get me wrong, though. America produced the most legit fake metal band ever with Dethklok. A fair amount of their fake music is in my hacking playlist, rivaling only In Flames.
An interesting aside, my running playlist was originally this, as 130bpm is the right cadence for my stride. Later I worked out it was great for hacking as well.
Fun fact: they are not too well respected in prog metal circles. They (I am told) have trouble playing their own songs live. Unlike the godfathers of the scene, Dream Theater, who the crowd cheers when they mess up.