This is a demo for converting an MP3 to MIDI. BUT a big disclaimer...this worked for me with 1 instrument. I recorded live piano music into a digi-recorder. Then processed the MP3. This process is a handy way to transcribe music (see limitations).

This Instructable requires that you install two freeware applications. This is a Windows-centric project (sorry to non-Win people). It may be possible to do this Instructable on another O/S. There are some optional applications for printing the MIDI file as sheet music or for editing the MIDI file.

One thing I don't get into is how rip one instrument/voice from an MP3 with more than one. If you know how to do this please post an Instructable. I would love to be able to isolate an instrument or voice from an MP3.

LIMITATIONS
As mentioned above the MP3 can only have one instrument/voice.
The sheet music is often a more complicated version than you expect. You may be able to quantize the MIDI file to make it more readable.
The sheet music only displays a treble or bass clef not both.
(Please report limitations that you discover.)

Step 1: Install Software (mostly Freeware).

Download and install the following Windows software:

Audacity (v 1.3)
Audacity is a really great open-source software. It is handy if you want to record via your laptop/desktop to MP3. It has a lot of effects.
http://audacity.sourceforge.net/download/
OR get a Portable version at...
http://portableapps.com/apps/music_video/audacity_portable

Amazing MIDI (v 1.7 )
This is also very cool software. It takes a single instrument input as WAV file and transcribes/converts the music to MIDI file. This software has not been updated since August 20, 2003.
http://www.pluto.dti.ne.jp/~araki/amazingmidi/

(Optional)

MIDI Notation (v 2.1.2)
This software takes a MIDI file displays it as sheet music which can be printed.
http://www.notation.com/DownloadNotationPlayer.htm

Anvil Studio (v 2007.12.01)
This software takes a MIDI file and allows the user to edit the music. The basic software is free but you can add on some components for a modest price.
http://www.freewarefiles.com/program_6_70_7660.html

I included the version number for the software that I used. In other words, I got this Instructable to work using these versions. But it is usally better to work with the latest stable version.

Step 2: Audacity

AUDACITY
Convert MP3 to WAV

Launch Audacity
File > Open
Load your MP3 file.
File > Export as WAV

You can play around with Audacity in order to get a shorter clip or try out some filters. But on your first pass, I suggest doing the basic conversion (MP3 to WAV).

Step 3: Amazing MIDI

AMAZING MIDI
Convert WAV to MIDI

Launch Amazing MIDI
Tone File
Map this to a WAV file that will be serve as your instrument. Amazing MIDI ships with a piano and sine wave but you can find some other WAV files to use.
Input File
Map this to the MP3.
Output File
This will autopopulate when you enter the MP3 file. Modify as needed.
Transcribe
Run Transcribe (from the Menu). Before it runs the conversion it will display some settings. I have not tweaked these; I just used the defaults.
Voila!!! Your MIDI file should appear where it is mapped in the Output file

Step 4: Optional

OPTIONAL
Launch Notation Player (Player.exe) and map to your MIDI file.
Now you can use Notation Player to view/print the MIDI as sheet music.
You can also play the music and follow along with the sheet music.

***One problem I have had is piano music is displayed with one staff. You can toggle between treble and bass clef but not both. I think this is a limitation of Amazing MIDI.

Step 5: Optional 2

OPTIONAL 2
ANVIL STUDIO
Launch Anvil Studio and open your MIDI file.
In Anvil Studio, you can modify the music. It also has a bunch of features to play with.