Regarding musical box vs. music box.  There is more to say than I have
time for at the moment.  Suffice to note that the terminology has
historical, cultural and technological origins.  However, there are also
the grammatical ramifications (not often enough that we can get 9
syllables out of just 2 words).  For a box to be music isn't possible.
That a box is musical conjures yet other possibilities, and problems.
Perhaps our etymological friends would like to comment.  More later.