One website that converts uploaded audio files to the MIDI format is Bear File Converter. Applications that can perform the same service include Qmus2MID, WIDI Audio To MIDI, and AmazingMIDI.
Bear File Converter is a website dedicated to converting files to other formats. They can also convert documents and images. All conversions are free, although the file must be uploaded to the site, rather than linked to off-site. Furthermore, in order to perform a conversion, users need a reliable Internet connection.
The applications AmazingMIDI, Qmus2Mid and WIDI Audio To MIDI are free and do not need Internet connectivity to use. AmazingMIDI is Windows-only, but WIDI Audio To MIDI can be used on Mac operating systems, and Qmus2MID is for Linux.