With my phone I recorded the part of the tune I wanted by using the voice recorder function on my phone and holding my phone near my stereo system speaker whilst playing the tune. Then just set that as my ringtone.
Things may have changed since last I contemplated getting a unique ringtone on the phone. But when I did try a few years back, the mobile needed a file that had the .MID extension to the name. The problem with these MIDI files don't hold the waveform information like most sound files do, but information on what instrument voice plays at what tone for how long. It was next to impossible to convert a "normal" sound file to that format; so I gave up.