Files with an MP4 extension are usually video files with accompanying audio, but some contain only audio. MP4 video files can be converted into MP3 audio files for use on personal digital audio players or other purposes. MP4 audio files can likewise be converted to the very popular MP3 format, which is much more widely-supported by both audio software and digital audio players.
If running Ubuntu or another Debian-based Linux distribution, go to System > Administration > Synaptic Package Manager. Otherwise, see Step 5.
Type "Sound Converter" into the Quick Search field. Find soundconverter in the list that populates, then click the empty checkbox next to it. Click "Mark for Installation," then click "Mark."
Type "gstreamer" into the Quick Search field. Sort the list by package name, then find gstreamer0.10-plugins-ugly-multiverse. (The number in the file name is a version number; if that number is different, that is okay.) Click the empty checkbox next to it. Click "Mark for Installation," then click "Mark."
Click the prominent "Apply" button near the top of the window. Synaptic Package Manager will install Sound Converter and the necessary plugin.
If not running Ubuntu or another Linux distribution that uses Synaptic Package manager, you will need to download and install both Sound Converter and the plugin that enables MP3 functionality. See Resources for both a link to the Sound Converter download page and a link to a page with details about obtaining the plugin.
Once Sound Converter and the plugin are installed, run Sound Converter. Click the "Add File" button near the top. Browse to the file you want to convert, then click "Open."
Click Edit > Preferences. Under "Type of Result?", choose MP3 Format. You can adjust the quality of your file, if you choose. Click "Close."
Click the prominent "Convert" button at the top. Your MP4 file will be converted into an MP3 and will be waiting for you in the folder you chose.