Things You'll Need
Instructions
Acquire AAC files. Use your preferred software to download files in the AAC format. These files are available, but nowhere near as widely as MP3.
If you cannot find the file you want in AAC format, you can use iTunes to convert the files. In iTunes' Preferences menu, open the Importing Settings tab. Open the Import Using tab and select the AAC settings you wish to use. Insert or import the CD or MP3 you wish to convert to AAC. Right-click on the track to be converted and select "Create AAC Version".
Load the AAC files onto your SD card. Navigate to the folder where iTunes saves converted music (by default, this folder is called "iTunes Music"). Insert your SD card into your computer's SD card reader and copy the AAC files onto the SD card.
Eject the SD card from your computer and insert it into your Wii. You now have the option to use your Wii to play any AAC files stored on your SD card.