What you (as not japanese reading guy) are trying to do is to read a japanese book in your pc that have mixed pages (without page numbers), without index and your pc can't show you japanese characters.
Just a mission impossible!!!!
Let me get concrete:
As you maybe know, a pc (or game console) can actually understand only 0 or 1.
A rom that you download from the internet is just a sequence of many 0 and 1 (some hex editors can show you the file in this form as well).
To not have long sequences of 0 and 1, this 0 and 1 are "groupped" in a special way that give you hexadecimal (you can find on the net how this "groupping" works).
So now you have a nice screen of hex numbers on screen and you and the pc (or game console basically don't know what to do with them).
Let's take a melon, a piece of paper and a knife as example. Make a cut in the melon so that you can insert the piece of paper and then push the knife completely inside the melon. You will now have a melon with a piece of paper and a knife inside and it's actually not clear where is what....this is you and your screen full of hex numbers.
What will happen if you give this melon (with the knife and the piece of paper inside) to somebody that never saw a knife and a melon???
Probably not much; he will need instructions in what to do the problem is that you and that person don't speak the same language but fortunatelly, in the piece of paper inside the melon, there are the instructions in how to use the knife, how to cut the melon and that the melon can be eaten.
The only thing you can do now is to point the guy to the piece of paper in the melon hoping that he will take it out of the melon and read it. The same is happens in the pc (game console)...they try to communicate till the pc finds the piece of paper (the program part) that it can understand.
Now, the guy reads in the paper that there is a knife inside the melon so he has to search for it and extract it.
Exactly the same, the program tell to the pc to go search somewhere inside that screen of hex numbers for something to use.
Now your guy is armed to eat the melon but he still need to cut a piece out of it so he continue read the paper that tell him that the melon need to be cut before he can eat it. The program to exactly the same with those hex numbers, it cut it into pieces and "eat" them, the only difference if that your guy has only 1 mouth but a pc has many mouth (the main processor, the sound processor, the graphic processor).
Now you probably start to understand that the same hex number on top of the rom, in the middle or at the bottom are in your eye all the same but not for your pc, it can be a piece of the melon, a piece of the knife or a piece of the paper.
The only way to find out what each of those numbers is, is a long work of trace and mapping the rom.
A good map for example is the one you will find here.
This is the top of the iceberg, even if you got a map like that before, you still need to find out how each part is stored so that it can be worked on by the program and "tasty" for the mouth that will eat it (the correct format for the hardware).
Now, you figured all of the above out and are ready to insert a new song but moment...your song is 10 second longer than the one that was inside that part before...what you do now?? Just cut it?? If you not do that, you will probably overwrite the next part in the rom (can be everything) and probably make the rom unusable (if you are lucky, you will only have the last 10 seconds of this song at the beginning of the next song).
What you really need to do is goto the program (piece of paper) and move everything after that song you want to insert, 10 seconds later but ...wait how many KB are 10 seconds of music?? You will need to know your audio format well to answer that...and did I forget something?? You and the pc don't talk the same language so how to modify the program???
By the way, the same apply to the graphics.
Still with me?? So what we conclude now??
Hexadecimal knowledge is required in any case but you will do absolutly nothing only with that.
You will need to learn ASM as well (to change the program), all graphics, audio formats in the game you are hacking, probably C#/C++ or any pc programming language so that you can convert the between formats, to manage all changes of offsets, to see what you are actually doing and finally alot of patience and time!!
I suggest you to read this
and maybe you will understand what awsome piece of hacking you have in your hands with those hacks.