MIDI
From Mario Fan Games Galaxy Wiki
MIDI, or Musical Instrument Digital Interface, is a musical format that sends song data directly to the sound card to be synthesized and relayed back to the user. MIDI is commonly used in fangames due to it's extremely small file size compared to WAVE and MP3 formats. However, MIDI sounds tend to be highly computerized and unrealistic. While most new game consoles (such as the Nintendo DS) have different sound cards to make up for this, personal computers still remain sounding highly computerized.
Due to the low-quality music MIDI usually produces, some MFGGers have begun using the OGG format in their fangames.