Can someone give me a simple tutorial on how to view and edit model\texture\sound files from a 3ds rom, and have another 3ds rom with the rom hack changes? I tried to follow a tutorial and most of the links are dead, and they do not work.
To extract and recompress the game's romfs (which contains those resources), you can follow the "Romhacking a CXI" in my signature (or use Asia81's PackHack if you are fine with a slightly less accurate rebuilt game)
After you have the files from the romfs extracted, though, it's all game specific - while there are some semi-standard formats, it's not mandatory for the game's developers to have used them!!
You'll need to figure out what formats are used to be recommended the right tools - assuming they exist for that format!