If you have some MP3 files that you want to convert to multi-track MIDI files using open source software, you can do it with the Audacity program. Open source software is free software that makes its source code open for everyone to see, edit or change as they see fit.
Go to the Source Forge website and download and install Audacity. There are separate download links for Mac and Windows computers, so be sure to click the correct one.
Launch Audacity. Click "File" and select "Open" to open your MP3 file in Audacity.
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Click "File" and then "Export" once the MP3 file is open. This will open the "Edit Metadata" dialog box. Click "OK" to move to the next screen.
Locate the "Format" field and click on the pull-down menu. Select "Other uncompressed files" as your output type. Click the "Options" button to the right of this field. Select your MIDI output type and click "OK," then click "Save" to start to convert MP3 to MIDI. A screen will pop up as it converts and you will see a progress bar on the screen. Once the conversion is complete, the screen will disappear. Look for the MIDI file in your Audacity folder.