To convert a song's file format:
1. Open iTunes Preferences.
Windows: Choose Edit > Preferences.
Mac: Choose iTunes > Preferences.
2. Click the General button, then click the Importing Settings button in the second section of the window.
3. From the Import Using pop-up menu, choose the encoding format that you want to convert the song to, then click OK to save the settings.
4. Select one or more songs in your library, then from the Advanced menu, choose the following setting(The menu item changes to show what's selected in your Importing preferences):
- Create MP3 version
If you haven't imported some songs into iTunes yet, you can import and convert them at the same time. This will create a converted copy of the file in your iTunes Library based on your iTunes preferences. To convert all the songs in a folder or on a disk, hold down the Option key (Mac) or Shift key (Windows) and choose Advanced > Convert Import preference setting. The Import preference setting will match what you chose in step 3. iTunes will prompt you for the location of the folder or disk you want to import and convert. All the songs in the folder or on the disk will be converted. Note: Some purchased songs are encoded using a protected AAC format that prevents them from being converted. iTunes Plus purchases are not protected and can be converted.
The song in its original format and the newly converted song appear in your library.
Additional Information
About compression
When you convert a song, some data may be lost due to the way certain formats compress data. For this reason these formats are sometimes called "lossy" formats. The advantage of using a "lossy" format is that the file sizes are much smaller, which means you can store more songs in the same amount of disk space. The disadvantage is that the sound quality may not be as good as the original, uncompressed format. Depending on the song, the speakers or headphones, and the player you use to play the song, you may not be able to tell the difference between a compressed "lossy" song and a song that is not compressed.
Once a song is compressed (meaning some of its data is lost) you cannot retrieve the data by uncompressing it. If you convert a song from a "lossy" format to a uncompressed format, the quality of the song will not improve and the file will only take up more disk space. For example, if you convert a song in MP3 format (a compressed format) to AIFF (an uncompressed format) the song will take up much more space on the hard disk, but it will still sound the same as the compressed file. In order to take advantage of uncompressed formats you should only import songs using these formats.
Tutorial Source:
support.apple.com/kb/HT1550
