Unfortunately, to my knowledge, you can't, and here's why. Midi files are not music files at all; they are instructions to the midi synth in your computer to play certain tones. Because music files...
This isn't as simple as it sounds.. as it's almost like asking how to turn an apple into an orange. MIDI files only contain performance information. Tempo, notes played, their velocity and duration...
Converting Music Files.
There is a cool software called "mp3 cutter joiner" in which you can convert .wav to .mp3. It says that it's a cutting and joing program but you can use it as a converter...
It depends what format of ringtone your phone supports. If it is a newer model, simply copy the .mp3 file across to your handset. Otherwise it is probably a .mid file or polyphonic ringtone. There...
You can buy an add-on for Windows Media Player to do the conversion. I think it's about $30. Another way is to download a freeware program called Audacity. You can import the music in to Audacity....