I usually search YouTube for 'music for deep focus, concentration, memory,' or something along those lines. These videos typically last about 3 hours, making them ideal for uninterrupted work. I wouldn't play much else in the background because I'd be distracted by wanting to sing along or move to the rhythm.
Depends on the music. Classical, jazz, or some sort of the music that does not have lyrics, i can see how it could help focus on the subject. Also this depends on the subject you are studying.
Almost all research in this area has shown that problem solving and memory recall tasks are performed better in silence than with any kind of background noise.
Not all learning activities have to be while listening to music, lo. Playing music is recommended only for learning activities whose activities are to repeat the material, not for exams. ;)
Trickle