Open up JetAudio. Don't open it by opening up an audio file. Instead, just open the program alone, either by accessing it from the applications menu or from the Windows menu in the lower left corner.

Open up the conversion settings. This can be done by clicking on the second tab (labeled "Conversion" on the main window).

Select the file to be converted. Click the "Add" button in the top left and then select the file (or files) that you want to convert from MP4 to MP3. Then, click "OK."

Select what format you would like to convert to. This is done by using the drop-down menu in the lower left corner. In this case, you will want to select MP3 as the output format.

Select the file name and location. Just above the output format menu are boxes to select the new file's name and location. Type in what you would like for those, and then hit the "Start" button in the upper right.