It's crucial to remember the original composition is protected by copyright law. Most free MIDI files are (like learning, arrangement practice, or research). You should never use a MIDI file for a commercial project or public distribution without obtaining proper licensing from the copyright holders.
Unlike standard audio formats like MP3 or WAV, a file does not contain actual sound waves. Instead, it functions as digital sheet music, mapping out precise musical performance details.
Assign a music box, glockenspiel, or celesta VST to the lead track to give it a hyper-realistic mechanical music box feel.
Ensure the file contains both the melody (right hand) and the bass/chords (left hand). A monophonic file will only give you the single-note melody line.