Go to intelliscore.net to download a trial version of Intelliscore Ensemble's MP3 to MIDI Converter (a full version is $129 as of early 2010). Click the link that's near the top center of the page--this is the current version. Once at the download page, scroll to the bottom and make sure you have the proper system requirements.
Find the location where you saved the downloaded file and install the software. After installation, a prompt will ask if you if you want to view the ReadMe.txt file. Read the "For Best Results" section.
Open the program. A window asks if you want to download additional software--click "No." The next window will give three options. Click "Continue with Demo." (The demo is full featured.) A Help screen appears with the wizard layered over it in a separate box.
Minimize the Help window and follow the wizard instructions. Once you're at the end, the program will begin to convert the MP3 file according to the parameters you gave, that is, what kind of instrument ensemble the song is made of, whether you know what the instruments are, and what pitch you want the instruments to be in.
Close the Conversion Results window. A window appears asking how you want to play the file. In the background the SyncPlayer main window will already be open. Keep the first option chosen. Minimize the Help menu and click on the blue arrow at the top of it to play the file. A small player will pop up and you can choose to hear the MIDI file solo, together with the original, or just the original file.