I always saw "Rockbox" as a playful mash of a few ideas in one otherwise nonsensical compound word:
Rock - as in the action of rocking or shaking, in this case jarring the dust off a cramped, outmoded operating system and unlocking shiny new potential. Also meaning a popular music style associated with rebellion.
Box - not only reminiscent of "jukebox" when paired with "rock" in this way, but also a reference to the boxy nature of most MP3 jukeboxes.
I think maybe I think too much about this stuff...