I hate to say it, but there isn't a single piece of music I like in this whole game. It's very basic and monotonous and it drives me crazy. I have to play with the sound off and my MP3 player in, which is great for jogging and stuff, but for the rhythem excercises (step, boxing, etc) which I actually really enjoy, it's a lot harder. Would it be possible to make some downloadable music packs for the games. Say...dance, rock, pop, just random packs that would suit a few peoples taste that we can download and will make the game 10x more enjoyable. Heck, I wouldn't even mind if you made the only music pack R&B which I personally dislike, just to give me something different to listen to!

The short answer is no, it isn't possible. Specifically: if Nintendo had meant the music to be updateable / replaceable, there would be options in the game's menus to allow you to select the music you want, as you find (sort of) in Walk It Out / Step To The Beat. (Even there, you can't add new music, just select the particular tracks you want to hear.)

The even longer answer goes something like this: The game is delivered on a non-writeable medium that looks a lot like a CD or DVD, but is most definitely neither of them. (By making it something else, Nintendo avoided having to license the trade-secret technologies for CDs and/or DVDs.) The practical result is that reading the disks on another type of machine, such as a PC, won't work, and you would not be able to update the content of the disks as DVD writers can't write Wii-format disks. So burning a new disk isn't an option. Writing music files to the Wii's internal storage isn't feasible either. A Wii 'block' is about 128K bytes (a 1GB SD card holds about 8000 blocks, so a block is about 1/8000 of 1GB, or around 128KB), so a normal 256Kbps MP3 file eats one block for about each 4 seconds of music, i.e. 15 blocks per minute. So, regardless of the practicalities of downloads and accessing them from the non-existent menus in-game, storing the music on the Wii is a non-starter.

The best I can suggest is to find music that matches the rhythm of the music for the step, boxing, etc. Other than that, there isn't much you can do.

Keep in mind, though, that these are only mini-games, and that the music isn't meant to last all that long. Myself, I rather like the music, and I have seen a very favourable review specifically of the music for WiiFit Plus, which pointed out not only how well it fitted in with the theme of each game, but how the music builds with the success of the player. For instance, you might have noticed in the Rhythm Kung Fu game that each time that extra instruments are added as you perform more perfect matches of the moves. Try listening again with this in mind and see if your opinion changes. I would point you at the review which is on line, but it's in French and requires a subscription.