For your modifications:
Sufficiently modifying the levels can be significant depending on how much you change them.
Making the background more interesting can be significant as well just be sure to do more than just adding in one or two sprites.
If you make your own music that would be a significant modification but using music from someone else would not be a significant modification on its own. So if you did that you would need to do some things in addition to it to get full points.