Obtain a freeware audio editing program online. (See Resources for a list of freeware audio editing program downloads.)
Click the "Save" action in the program's file download box to begin installing the audio editing program to your computer's hard drive. Opt to run the program upon the completion of the installation.
Press the "Ctrl" and "O" keys on your computer's keyboard to locate and select the MP3. Click the "Open" action to load the MP3 file into the audio editing program. The song's digital waveform will appear in the program's window.
Drag the mouse over the entire digital waveform to highlight it. This will indicate that the selected region is what you want to play backward.
Click the "Effects" button and select the "Reverse" action. The MP3's digital waveform will take approximately 30 to 60 seconds to load backward.
Click the "Play" button or icon along the top or bottom of the audio editing program to play back the reversed digital waveform of the MP3. The entire MP3 file will play backward.