MP3 is still the standard file format for music files. iTunes 11 likes to work with AAC files by default, but there is a way you can tell iTunes to convert music to MP3 format. Just follow these steps.
1. Select the Menu icon in the upper-right corner, then select Preferences.
2. Under the General tab, select Import Settings…
3. Set the Import Using setting to MP3 Encoder and click OK, then OK again to save the changes.
4. Now the Create MP3 Version option will be available on any song you right-click on. Selecting it will make iTunes convert the track to MP3 format.