Well technically there is no way to "convert" because there is no "sound" in a MIDI file. What I think you want to do is "render" it. QT pro can do that and an app like ProTools will let you bring in the MIDI tracks you can then assign them to an instrument and record the instruments out put on to audio tracks. The quality of the result file in all cases is very dependent on the quality of the MIDI instrument playing the file.