Download and install the free audio editor Audacity, if you do not own a commercial program. If you do own a commercial program, skip to step 4.
Download the LAME audio encoder to install in Audacity.
Click the Lame audio encoder file twice to install Lame for Audacity.
Import or drag the audio file you would like to convert into the audio editing application.
Select the "Export," "Export Audio" or "Bounce to Audio" option in the "File" menu, depending on which application you are using.
Open the "Export" options selection in the "Save As" dialog box. Select MP3 as your audio format. Identify and select any additional options you would like and save your MP3 file by pressing "OK." Additional options typically include bit rate, and options between variable and constant. Variable bit rate will analyze the audio file to find areas where bit rate may be lowered. This will decrease the size of the file without sacrificing quality.