Open iTunes.
Click “iTunes” and select “Preferences” on a Mac, or click “Edit” and then click “Preferences” if using a PC.
Click “Import Settings” located next to the “When you insert a CD” box.
Select “MP3 Encoder” in the drop down box next to “Import Using.” Doing so will dictate what format iTunes will default to when importing or converting an audio file.
Click “OK.”
Click “OK” again to exit out of iTunes preferences.
Right-click or control click the song title of the MP4 that you would like to convert into an MP3. Select “Create MP3 Version.” After doing so, iTunes will create an MP3 version of the song directly below the MP4 version.