I was under the impression that the Korg's only recorded to a stereo file... Hence, if I understand the initial poster, you CAN'T 'mix' the file on a computer. It is already mixed!

You can master it (and it WILL need it!), but you can't actually MIX it there... (change levels, EQ, compression reverb, etc. on individual sounds or voices)

Nothing short of the software arrangers will do a multitrack recording on an arranger, AFAIK, which is what you would need to MIX afterwards on a computer.