Sound effects the Mario Kart Wii system and UI sounds ripped directly from the game. Credit to '☀ ＡＶＤ MKW' for posting all the MKWii sound effects onto his YouTube channel, and of course Nintendo for creating the sounds.
I previously posted the Item SFX from the game too and will be posting part 2 of the system sounds as there were quite a lot and it exceeded the max file size.
All the sounds have their Japanese names but you should be able to guess what some of them are likely to be. You can always rename these afterward and feel free to use the sounds for games or animations, as long as credit is given where it is due.