Wow! Great question. I would like to think that Holden would listen to music that is original in topic and genre. He would not, as stated in #2, go for anything pop or overly played. He would, most likely, stick to genres that were not rip-offs of other bands or artists. Arguably, I could see him listening to something like The Eagles or The Hollies (On a Carousel), on his IPod, while watching Phoebe on the carousal.
On the other hand, I could see Holden listening to death metal (or other metal genres which speak to anger, hate, atheism). I could see this based upon Holden's own internal conflicts so as to find an outlet.
I do find the post about bars relevant. Holden does spend a lot of time in bars in the novel and, as out of character as it seems, likes to dance. So, I guess I could see him in a bar grinding to Lady Gaga or Spears as well.
This is actually a pretty hard question! Way to go!
Good points, but the key is that he desperately wanted to appear different. So what kind of music would he listen to when no one was observing? I agree that he would not listen to the "contemporary pop" type of performers who seem to sound the same and sing the same. He would probably listen to bands that have a unique style or some unique message. With the suppressed anger and depression that Holden has rap and hard rock or heavy metal might appeal to his emotions.
It might be easier to start with some songs Holden Caulfield wouldn't like. Holden Caulfield likely wouldn't like anything "phony" which would likely include most up and coming artists that got their start on American Idol, anything pop and/or popular among teenaged girls (Justin Beiber comes to mind), or anything overdone, like most of the new hip hop artists.
It is hard to imagine a present-day Holden Caulfield, but considering he spent a large part of the novel in bars, pretending to be older than he was, I imagine he'd be into obscure indy- bands and alternative music, as well as classic rock, anything influenced by blues and/or jazz and big band music. Even now, I imagine he would be the type of kid to go through old LP's in his attic and get really interested in some of the good music of older generations.
I don't see him as "goth" or "emo" or any of the other current high school labels, as he desperately wanted to appear to be different (and above) everyone else.
I can see him listening to Vampire Weekend, The Smiths, Belle & Sebastian. Those type of indie bands that are moderately preppy with a sense of emotion and actual depth. Sort of like Holden himself.