TS-AudioToMIDI

TS-AudioToMIDI is a tool for automated recognition of polyphonic
music. Music recognition is what musician does when playing
music by ear and then recording it to staves. This program
analyzes input waveform and tries to detect what notes were
played.
TS-AudioToMIDI is able to both recognize pre-recorded audio
files and perform recognition in real-time.
Recognizing pre-recorded music provides the possibility to
convert digital sound (mp3, wav, CD tracks etc.) to MIDI
format.
Real-time recognition is a process performed in real-time,
without noticeable delay. Detected notes can be either recorded
as MIDI file or played in real time, or both.

As MIDI file is, in fact, a layout of music, you can change its
arrangement in contrast to digital sound. Instruments,
number of voices or melody itself can be easily modified using
any third-party score editor, which allows creating remixes or
variations.