Jump back?!?! This is a neat thread!
Well my guess for Hermione is probably some classical or jazz stuff when she does her homework and then I would guess some sort of a "secret" music...like Clay Aiken or something.
As for ickle ronnie perhaps I see some sort of a radio music, the popular kind stuff, but not rap or hip hop or what ever. You know like Bare Naked Ladies, 3 Doors Down, Good Charlotte, Train, Dave Matthews Band, All-American Rejects, Simple Plan...all of those pop rock, pop punk, and what ever pop bands that are on the radio. I just seem him liking that kind of stuff since he reminds me of a friend who likes that kind of music.
Harry, now harry, he has some emotional issues. That's why I think he is an emo kid. He's got black shaggy hair, black rimmed glasses, wears hand-me-down thrift shop style clothes, and rejects society's norm. He also has the required teenage angst, which I believe he'd express through emotional punk rock music and post-emo indie rock. I see him listening to bands such as Alkaline Trio, The Get Up Kids, AFI, Sunny Day Real Estate, Thursday, Saves the Day, Dashboard Confessional, and Hot Water Music to convey his emotions.