Just use MiiMusic... Google for it.
Its a site you use on ur PC ti install 2 things on your pc.
After that you can make a playlist with that program, and the other one will open you your own IP server..
After that you go on your wii to the same site, but this time wou will see an other version of the site, cause it recognize the Wii IP
after that you click on open my playlist.
Click on your playlist name, and then it will stream all your music from PC to WII. For free.
a much easier way for playing mp3 on Wii because you don't need to have a SD card, or delete a lot of mp3's because your Sd is full ;P