MIDI files are not actually audio files. MIDI stands for Musical Instrument Digital Interface, and the files contain data as to what notes to play on what instruments, how loud each note is and how long it is held. Conversely, MP3 is an audio format. Usually, you cannot convert a MIDI file to an MP3 file, since they are not both audio files. You have to convert MIDI to WAV and then WAV to MP3, but iTunes has cut out the middleman. With iTunes, you can convert MIDI to MP3 on your Mac.
Download the latest version of iTunes for your Mac (see Resources) if you are not already running the latest software.
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Open iTunes. iTunes is located in your Dock or in your Applications folder inside of your Hard Drive.
Go to the iTunes menu in the menu bar and scroll down to Preferences. A new window will open.
Click Import Settings. Another new window will open. Select MP3 Encoder from the scroll down menu entitled Import Using. Select the quality from the Setting menu. Click OK in the lower right hand side of the window.
Select Add to Library from the File menu or press Command O. A window will open allowing you to choose a MIDI file. Select the MIDI file you would like to convert to MP3. Press Choose in the lower right hand side of the window. The file will convert to MP3.
Find the new MP3 file in iTunes by using the search function in the upper right hand corner of the iTunes window.
Some sound quality may be lost when converting MIDI to MP3 on your Mac with iTunes.