The file you're looking to modify is called EditSound.bcsar . This is an archive that contains all the components to that Mii Maker main theme song. It has a bunch of .bcwav files.
This is not a straight up single file music swap. The 3DS does sound track by playing each of those .bcwav files as if recreating the music by individual instruments.
If you want to add the trap remix portion, you have to recreate that song from scratch with addition .bcwav files matching those bass drops, etc.
It will suck super bad doing this even for someone who knows the ins and outs of 3DS title decompiling and recompiling.