Neku's headphones are probably the most significant part of his attire. It is the one article of clothing he wears that has been brought up multiple times throughout the game, which is also the one part of Neku that's always looked down upon besides his cold personality.

Never once until the end of the game had Neku removed his headphones. One reason why he never does is that aside from blocking out others, he can easily function with music. Some people have tendencies of playing music or turning on the television when it gets too quiet. Neku can also draw inspiration from the lyrics or the beat he's listening to.

As for the music, I do think that they're the many tracks that can be bought through the game at AMX, Cyco Records, and Towa Records. While standing still on a map, Neku can be seen bobbing his head to the music with a hand on his headphones.