Mario Kart Wii help/doubt

For Wii? There are whole custom tracks you can download and make. These are mostly presented in complete ISO packages. Google is your friend here due to forum rules about piracy.

 

You're right, it would play Piranha Plant Pipe music on Koopa Cape in that case. As to my knowledge, there is no easy way around this.

 

Custom tracks use Nintendo tracks as a base, because to my knowledge, you can't write new course entries into MKWii, or if you can, no modders are sophisticated enough to have figured out how yet. I would assume it's hard-coded.

When you play Piranha Plant Pipe, you're really playing a heavily edited version of Koopa Cape. So when the game loads PPP, it's really being told to load this heavily edited version of KC that is on your SD card, and so it uses the KC music file. There's no way around it. If you want to change the custom track music, you have to change the Nintendo track music.

 

I'm not completely sure if it would work, but here's an idea that would take a lot more effort than it's worth:
Add triggers to custom tracks that switch it to other audio tracks in the audio file, and make extra audio tracks for the new music
(Courses such as Koopa Cape, Toad's Factory, and Bowser's Castle use these for switching to inside variants of the song)
The brsar would also have to be edited, I believe
I don't know for sure if it would work, and it's probably not at all worth the effort, but it's an idea