Convert MP4 to MP3 files by using SUPER on Windows and iTunes on Mac OS X systems. ITunes comes installed as a part of the Mac operating system, but users must download and install SUPER separately; SUPER is available for free.
To convert MP4 files in SUPER, right click anywhere on the SUPER window, and choose Add Multimedia Files from the list. Select the MP4 files you want to convert, and load them into the application. Select MP3 from the Output Container drop-down list, and click Encode. SUPER also allows you to fine-tune some of the encoding settings.
To perform the conversion in iTunes, click iTunes from the menu bar at the top of the screen, and then click Preferences. Click the General button, and open Import Settings. Choose MP3 as the encoding format and click OK to save the settings. Open your iTunes Library, select the MP4 file you want to convert, and click File from the menu bar. Click Create New Version, and choose Create MP3 Version.
Once you've changed your Import Settings, you can convert all MP4 songs in a folder by holding the Option key and then using the Create New Version option. Apple warns that older songs may be ineligible for conversion due to the Protected AAC encoding used in older songs.
Unlike MP4, MP3 does not support video encoding. As such, any MP4 video file you convert only retains audio data.