I don't have time to go into detail, but you don't actually "convert" midi to audio. (Some expensive programs can do that, but that's a topic for another day)
You will have to "record" the audio from the keyboard outs directly INTO the pc through the "line input".
Power Tracks Pro should be able to do what you want, but that's on the lap top only, right? Maybe you can download a free program that records audio and converts wav to mp3 or wma.
Sorry to be so vague, but I'm on my way out the door. Good luck !