I am not entirely sure what you are heading for here.
I am not sure what you mean by level code in the background picture. Can you give an example? I don't know if you mean level text, scrolling backgrounds, some kind of game where there is a foreground and background part of the level or similar.
Music on NES games is usually completely custom from the ground up, and can vary a bit if you head into some of the more exotic mappers and FDS stuff.
If you want to make it play different tracks you will have to take each game as it comes and work with that. Many ultimately could boil down to a pointer change or quick tweak of a command (if you can select tracks easily in a sound test screen, which many games could, then it is probably going to be this) but the trick is getting to that point. For the NES I don't really have an easy way of doing that without playing with assembly, which if you still don't care to even mess around with basic hex approaches is going to be right out. If you want to edit tracks you are probably still going to run into the need to program things (composes in the early home computers through 16 bit era were often accomplished programmers in their own right).
Or do you mean you want to make some kind of quasi NSF file (NSF files being what those emulators that play audio tracks from games are, them tending to be NES ROMs tweaked to leave the sound engine and and the sound with little else, or custom things from the ground up) such that your emulator knows to play a MP3 or something.